Aa

Hawaiian name for a type of lava that is chunky and typically slow moving.

The image at the left is a flow of aa lava over another type another form of lava called pahoehoe. Image can be found at Galapagos Lava Fields and Flows.

Abandoned Channel

A stream channel that no longer has water flow.

Abaxial

The surface of any structure that faces away from the main axis, such as the bottom part of a leaf. Compare with adaxial.

Abdomen

The central cavity of vertebrate animals that contain the digestive, or visceral organs, like the stomach, intestines, and the liver.
The posterior portion of an arthropod.

Abell Catalogue

A listing of over 2700 rich clusters of galaxies, compiled and published by George Ogden Abell. This catalogue is used to help define objects to qualify as an Abell Cluster.

Abell Cluster

A specific type of galaxy that can be found in the Abell catalogue. Objects that are considered as an Abell cluster are typically have a very dense concentration of galaxies, usually about 30-50 or more.

Abell radius

A region of space that has a radius of about 2 megaparsecs (or an area of about 12.6 megaparsecs) that contain at least 30-50 galaxies which they can then consider as an Abell cluster.

Abiogenesis

The formation of life from non-living, possibly organic matter.

Abiotic

The non-living factors in an environment, such as climate and geology. Also refers to processes and substances.

Ablation

Generally, the melting or wearing away of material. In geology, can refer to ice, snow, or rock.

Abomasum

The fourth stomach of ruminants like the cow. The image to the right (courtesy of Seattle PI) shows a simple cartoon of cow digestion, and is labeled as follows:

The cow eats some grass.
The grass is passed onto the rumen and the reticulum where it is softened.
The softened grass is passed back up to the mouth as cud, which the cow chews up and then passes to the third stomach, the omasum.
Nutrients are absorbed in both the omasum and the fourth stomach, the abomasum.
Finally, the stuff leftover is expelled as waste.

Aboral

Away from the mouth.

Abrasion

Any process that involves the wearing down of a substance or material, especially rocks.

Abscisic Acid (ABA abscisic, dormin)

A hormone in plants that keeps leaf buds from growing, and helps make leaves and fruits fall (senescence).

Abscission

The process of shedding an old or mature organ in plants, for example a ripened mature fruit, or a dead leaf.

Abscission layer

In plants, this is the layer of cells where abscission takes place. In deciduous trees, an abscission layer forms between the petiole of a leaf, and the stem. In autumn, cells in the abscission layer will breakdown and eventually disappear, leaving the leaf attached to the tree by a few transport tubes. This connection is then broken by weather, and results in a falling leaf. A similar process occurs in fruits and some flowers.

Abscission zone

The area of a plant where the abscission layer forms and where the structure separates from the parent plant.

Absolute age

A definitive age given for geologic material (plus or minus a given margin of years) independent of relative geologic age with other geologic material. This age can be determined through a variety of ways, including radiometric dating and dendrochronology. Compare with relative age.

Absolute magnitude

The apparent magnitude of a star if it were a distance of 10 parsecs from Earth. Astronomers denote absolute magnitude with the symbol M.

Absolute plate motion

In the context of plate tectonic theory, the motion of a lithospheric plate with reference to a specific point. A very good example of absolute plate motion comes from the formation of the Hawaiian islands. The Hawaiian Islands are located in the relative middle of the Pacific plate, and the island chain gives an idea of how the Pacific plate has moved over time.

Absolute pollen frequency

A method of expressing the amount of pollen in sediment that gives absolute numbers for every representation of species. Compare with relative pollen frequency.

Absolute temperature

A measure of temperature on an absolute scale. The typical unit of measurement here is K (for Kelvin).

Absolute zero

The coldest possible temperature, usually depicted as 0 K. Equivalent temperatures: -273.15° Celsius, or Fahrenheit.

Absorptance

The capacity of a material to absorb certain forms of radiation.

Absorptance band

The range of wavelengths of radiation an absorbant material can absorb.

Absorption coefficient

A measure of the decrease of radiation as it passes through different substances.

Absorption line

A dark line or a bar that appears on a stars spectrum, which indicates elements that are on the stars surface, and absorbing radiation. These lines are a sort of chemical signature that help identify the chemical composition of the star.

Absorption spectrum

A graphic representation of the amount of electromagnetic energy that is absorbed by a substance.

Abstraction

The removal or extraction of something. In geology, this term refers specifically to water that is removed from a well, river, or reservoir.

Abyssal

Term referring to the region of the ocean below about 1000 meters (about 3300 feet) and beyond the continental shelf.

Abyssal hills

Term given to relatively shallow hills that can be found on abyssal plains. These hills are typically only about 50-250 m in height, and a few kilometers thick.

Abyssal plains

Portions of the seafloor which are relatively smooth, and have a gentle gradient (typically a drop of about 1 foot every 2 miles).

Abyssal storm

A large surge of energy on the ocean floor that stir up large amounts of fine sediment.

Abyssal zone

The region of an ocean floor where the general depth is below 2000 m.

Accessory Nerves

In anatomy, the accessory nerves, sometimes also called the cranial nerves XI, are a pair of nerves that come from both the brainstem and the spinal cord. These nerves supply motor impulses to the throat muscles for swallowing and speech.

Accessory Pigment

A light-absorbing pigment that trap light at wavelengths beyond those used by chlorophyll. Examples include anthocyanin and carotenoids.

Accretion

The process by which a body of matter increases in size and mass by the addition of more matter. Generally accepted model of how planets form around new stars.

Accretion disk

A flattened region of matter and/or gas that develops around a central massive object, such as a black hole. In this artists rendering, the disk is formed around a white dwarf star.

Image can be found at Astronomy Picture of the Day.

Accretional heating

The heat generated by the kinetic energy of matter colliding with other matter during the process of accretion.

Accretionary lapilli

Little balls of volcanish ash that form by the accretion of ash around small particles or water. They range in size from 2-64 mm.

Accretionary wedge

An accretion of sediment that forms at certain plate boundaries. When one plate is subducted beneath another, sometimes the top layer of the subducted plate is scraped off by the subducting plate. This structure is the accretionary wedge.

Accumulation zone

Also known as the accumulation area. The area of a glacier where there is more mass gained than lost.

Acellular

Referring to any organism or tissue that is not divided into cells, such as plasmodial slime molds, and some fungal mycelia.

Acentric

Refers to a chromosome or chromatid that lacks a centromere.

Achene

A small, dry, single-seeded indehiscent fruit. Examples include pumpkin seeds, sunflower seeds (pictured at right), and strawberry seeds.

Achlamydeous

In flowers, something that is lacking petals and sepals.

Achlorophyllous

Without chlorophyll.

Achondrite

A type of stony meteorite that generally lacks chondrules, composed mainly of silicate material.

Achromatic

Without color. In astronomy, refers to a type of lens that refracts or reflects light without chromatic aberration.

Acicular

In leaves, a term that means "needle-shaped." Examples include pine needles.

Acid

Any substance that has a pH level less than 7, and can be dissolved in water. Compare with base.

A-class asteroid

A rare type of asteroid. They are most probably comprised of a non-mineral called olivine. These asteroids have a high albedo (from 0.13-0.35), and a reddish color.

Acoelomate

A triploblastic organism that only has a gut, and no other body cavities.

Acrocentric

A chromosome or chromatid in which the centromere is off-center.

Acromion

A bony process where the clavicle attaches to the scapula in mammals and mammal-like reptiles.

Image courtesy of Handball City: Shoulder Injuries.

Acropetal

A botany term that refers to a plant that grows and develops successively from its base (or oldest part) to its tip (youngest parts).

Acrosome

A membrane-bound section found at the head of sperm cells which contains lytic enzymes which are used to break through the outer layer of an egg, so that the sperm can inject its haploid DNA.

Actin

One of two proteins that are involved with muscle contraction (along with myosin). Actin is also an important component in the cytoskelton of many eukaryotic cells.

Actinomorphic (Botany)

Image of a buttercup demonstrating its actinomorphic quality.

In flowers, a term that means symmetric along more than one plane. It is equivalent to radial symmetry in animals. Examples include buttercups, pictured at right, and petunias. Compare with zygomorphic.

Activating enzymes

Enzymes that are responsible for attaching the correct amino acids to their respective tRNA molecules. Each amino acid has its own specific activating enzyme.

Activation

In eggs, its the series of biochemical and physiological events that happen after a spermatozoan penetrates the female egg during fertilization.

Activator

In biochemistry, a DNA-binding protein that helps to regulate gene transcription.

Active Immunity

An organisms immunity from disease that is acquired from some level of previous exposure, either through infection or inoculation. An example for humans is the flu shot, where multiple weak strains of the influenza virus are administered by a shot to help the body produce antibodies to fend off a stronger infection from a more powerful form of the virus.

Active site

A specific area on an enzyme where a substrate binds and a catalysis reaction takes place.

Active transport

The transport of molecules and ions across a cell membrane usually against a concentration gradient or an electrochemical gradient. For this to occur, an energy boost is needed, such as ATP hydrolysis.

View movie.

Actomyosin

A protein complex made up of actin and myosin found in muscle fiber.

Adaptation

From an evolutionary standpoint, an inherited or an acquired characteristic which has made an organism better capable of living in its environment, whether through physiological change, or through natural selection.

Adaptive radiation

Term referring to the formation of multiple daughter species from one or few ancestral species, usually in relation to a newly vacated or formed ecological niche. One popular example can be found with the popular finches of the Galapagos Islands, of which Charles Darwin made famous. Generally thought to be related to birds found in coastal South America, a few members of the South American birds made it to the islands, and diversified as a result of the varied climates and environments found within the Galapagos.

Adaxial

The side of a lateral structure that faces the main axis, such as the top side of a leaf.

Adenine

A nitrogenous base that pairs with thymine in DNA, and uracil in RNA. When you see a string of letters that are used to describe DNA, such as AGTCCGTTATATTGC, "A" stands for adenine.

Adenosine

A nucleoside of adenine that is bound to a ribose molecule.

Adenosine deaminase

An enzyme that converts adenosine to inosine.

Adenosine diphosphate

Also known as ADP. A nucleoside that is attached to two phosphate groups. ADP is formed when ATP "donates" a phosphate group during active transport.

Adenosine monophosphate

Also known as AMP. It is the nucleoside adenosine bound to only one phosphate group.

Adenosine triphosphate

Also known as ATP. An energy-carrying nucleoside that is made up of adenine, ribose, and three phosphate groups.

Adiabatic expansion

A type of expansion that happens without heat entering or leaving a system, like air expanding because of atmospheric pressure.

Adipose tissue

A type of connective tissue that stores fat cells (also known as adipocytes).

Adjuvant

A substance that is added to a vaccine to improve the immune response, such as helping lymphocytes divide faster.

Adult

A mature individual that can produce gametes.

Aerenchyma

In plants, tissue that contains large gas-filled spaces in between the plant cells. Typically found in aquatic and semi-aquatic plants, which helps provide buoyancy.

Aerobe

An organism requiring oxygen (O₂) for growth, or respiration.

Aerobic

Requiring oxygen (O₂) for growth, or respiration.

Aerobic respiration

Type of respiration that makes use of free oxygen and provides an efficient source of energy for aerobic organisms.

Aerotaxis

Term used in reference to an organisms movement either towards, or away from a concentration of oxygen as a reaction to its presence. Typically used in correlation with aerobic and/or anaerobic organisms.

Afferent

A term used to refer to movement from the outer part of the body, to a central part of it. Usually used in reference to nerves, vessels, etc. An example would be blood vessels moving from the arm towards the heart. Compare with efferent.

Agamont

Term referring to a mature adult that is not capable of sexual reproduction, and does not produce gametes. Compare with gamont.

Agamospecies

A term used to describe any species that can only reproduce asexually. Literally, it means "without gametes."

Agamospermy

Referring to any form of asexual reproduction that results in the formation of fruits and seeds.

Agamospory

Asexual formation of an embryo and the subsequent development of a seed.

Agar

A gelatinous extract taken from some forms of red algae that is used as a culture medium, especially for culturing bacteria. In the image to the right, agar is the colored material in the petrie dishes.

Agglutination

The clumping together of cells, such as red blood cells, that forms a linked network between the cells. Distinct from an aggregation.

Aggregation

Clumping together of cells, or organisms, without a physical link. Compare with agglutination.

Agonism

When two or more processes or systems cooperate. Examples include:

Drugs or hormones that work together so that their combined affect is multiplied.
The movements of paired muscles, such that one contracts while the other relaxes.
Agonistic behavior: An animal behavior that includes both aggressive and avoidance. The idea behind it is to look aggressive, while avoiding any actual physical violence, and can typically be seen in mating rituals.

Agranulocyte

A type of leukocyte that lacks granules in its cytoplasm.

Albedo

The measure of brightness for objects that don't emit their own light. The range is from 0, which is totally dark, to 1, which is completely reflective.

Albinism

A rare genetic disposition where an animal lacks the genes that produce the skin pigment melanin. An animal that has this condition is called an albino.

Albumen

Material found in developing embryoes, both plant and animal. In animals (birds and reptiles), it is the egg white. Albumen is made of the protein albumin, primarily, and provides nutrients to the developing embryo.

Albumin

Any of several proteins that can be found in both plants and animals, and are used to balance levels of salt and water, as well as transport small molecules.

Alcohol

An organic compound that has at least one hydroxyl group.

Alimentary canal

Also known as the digestive tract. Generally, a hollow muscle-lined tube that extends from the mouth to the anus of an animal. The digestion of food particles is done along this canal, with the introduction of digestive enzymes. Food in the alimentary canal is moved along by muscle contractions called peristalsis.

Alkaloid

A group of complex organic compounds which can be found in many plants. Examples include quinine and caffeine.

Allantois

A membranous sac that forms on the ventral side of the hindgut in embryonic amniotes. This structure combines with the chlorion to form the placenta. In some mammals, this can form into a urinary bladder.

Allele

One of at least two genetic variants on a chromosome.

Allele frequency

A term used to compare the amount of a particular allele is found within a population. The more often an allele is found within a population, the higher its frequency.

Allelochemic

Any chemical substance that is produced by one species that affects another

Allelopathetic

Any chemical substance released by one plant species which affects another plant species, sometimes in a negative way, such as inhibiting germination.

Allogamy

Cross-fertilization in flowering plants.

Allometry

The study of the relationship between the size and shape of any part of an organism in relation to the rest of its body. An example would be the size of a puppy's paws in relationship with the rest of its body, and how that relationship changes as the puppy grows.

Allopatric

A population of the same species or sub-species that cannot breed because of geographical separation, such as a mountain range. Compare with sympatric.

Allopolyploid

A hybrid between two separate diploid species, typically plants, resulting in a polyploid offspring.

Allotetraploid

An organism that is an allopolyploid that has four sets of haploid chromosomes.

Allotopic

Term used to describe closely related sympatric populations that inhabit the same geographical area, but live in different habitats within that geographical area. An example would be cichlid fishes from Lake Victoria in Africa.

Image can be found at Fish Profile.

Altricial

A term that refers to animals that are blind and helpess at birth. Compare with precocial.

Altruism

A behavior in animals where one favors the survival of others of the same species without apparent benefit to itself.

Alveolus

Any small, sac-like pit or cavity, such as the alveoli (plural form) in human lungs.

Ameba

Alternately "amoeba." Any of the eukaryotic protists characterized by their amorphous shape, and their use of pseudopodia that they use to move around, and for capturing prey.

Amensalism

A type of symbiosis where one organism suffers greatly, and the other is unaffected. An example of amensalism would be a sapling growing near a fully grown tree. The adult tree deprives the sapling of sunlight and possibly nutrients, so the sapling suffers greatly. In comparison, the saplings presence doesn't affect the adult tree at all.

Amino acid

Any of 20 different, but specific types of molecules that combine to form proteins.

Amino group

A functional group formed from ammonia (NH₃) that has lost a hydrogen atom, therefore becoming ~NH₂.

Amino sugar

A monosaccharide in which an amino group has replaced one or more hydroxyl groups.

Amitochondriate

Any of the eukaryotic protoctists belonging to either the Archaeprotista or Microspora, so named for their lack of mitochondria.

Amitosis

A form of cell division where the nucleus does not go through mitosis, resulting in an inequal amount of chromosome distribution.

Ammonotelic

Any animal whose chief nitrogenous waste product is ammonia (NH₃).

Amnion

A membranous sac that contains a developing reptile, mammal, or bird.

Amniote

A term that refers to any animal that develops as an embryo inside an amnion, either in an egg or within the mother. Reptiles, birds, and mammals are all examples of amniotes.

Amoeba

Alternately "ameba." Any of the eukaryotic protists characterized by their amorphous shape, and their use of pseudopodia that they use to move around, and for capturing prey.

Amoeboid

Anything that resembles an amoeba.

Amorph

Can be defined as:

An inactive gene.
Anything that lacks a specific shape.
A mutant member of a population that completely lacks a particular structure.

Amphibolic

A metabolic pathway that has both anabolic and catabolic reactions.

Anabolic pathway

A reaction in a metabolic pathway that synthesizes complex molecules from simpler ones, and usually needs energy to do so. Compare with catabolic.

Anadromous

A term that refers to fish that reproduce in fresh water, and live in sea or ocean waters. Salmon are a good example.

Anaerobe

An organism that does not require oxygen (O₂) for growth, or respiration.

Anaerobic

Referring to organisms that do not require oxygen (O₂) for growth, or respiration.

Analogous

Any anatomical structure, or behavior that is similar in two separate and unrelated species. One example is the different shapes of wings in bats, birds, and pterosaurs. All do essentially the same thing (provide lift for flight) and look fairly similar, but all three groups are unrelated. Compare with homologous.

Anamniote

Any vertebrate animal that does not develop in an amnion. As a result, these animals need to lay their eggs in water, or damp environments. Fish and amphibians are such animals.

Anaphase

A stage during meiosis or mitosis where chromatids or chromosomes separate and move to opposing sides of a cell. This phase follows metaphase.

Androdioecious

In flowering plants, having male parts on one plant, and hermaphroditic parts on another.

Androecium

The collective name for all of the stamens of a flower.

Androgen

Any of several types of steroids that promotes male hormone activity and maintenance of secondary male characteristics.

Andromonoecious

In flowering plants, a plant that has both male and hermaphroditic parts on the same plant.

Anemophilous

Flowering plants that are primarily pollinated by the wind. Compare with entomophilous.

Anestrus

The period between estrus. Also known as anoestrus.

Aneuploid

A cell or organism that has an abnormal number of chromosomes.

Angiogenesis

The process where new blood vessels are formed. This typically happens in a developing embryo, but can happen in malignant tumors.

Angiosperm

A type of plant that can produce flowers and encases its seeds in a protective coating (like a fruit).

Angstrom

A unit of measurement equivalent to 10^-10 meters, (or 0.0000000001 meters). The symbol used for angstrom looks like an A with a little circle above it (Å).

Animal

Any heterotrophic eukaryote that forms a blastula during development.

Animal pole

In a developing blastula, the end that has a lower concentration of yolk. It is opposite the vegetal pole.

Anisogamous

A condition where the respective gametes of a sexually reproducing species are not the same size, and/or move differently. An example would be human egg and sperm.

Annual

Any plant that goes through its entire life cycle in one season.

Annular

Ring-shaped.

Annulus

The ringlike structure on the stipe, derived from the partial veil.

Anoxia

Lacking in free oxygen.

Antagonism

Situation when two processes or systems are working in opposition to each other. Examples include:

Drugs or hormones that counteract each other, making each respective drug or hormone ineffective.
One organism that interferes with the development of another, such as some parasites.

Antenna

Paired appendages on the heads of most Arthropods, which perform various functions, such as sense of smell and taste, or swimming.

The front portion of an object or organism. Compare with posterior.

Anther

The portion of a plant's stamen that bears pollen.

Antheridium

The male reproductive structure in some fungi and seedless plants.

Anthocyanin

Image of leaves with a purplish color, demonstrating a form of anthcyanin.

A light-absorbing pigment that can give some plants and other organisms a blue, red, or purplish color.

Antibiotic

A generic term used to describe an organic compound produced by microorganisms that are either lethal or harmful to other microorganisms.

Antibody

A specialized protein(or immunoglobulin) which is used to fight off infection. Antibodies are produced when an antigen is introduced into the body. Specific antibodies bind to the antigen, which is sometimes consumed or destroyed.

Anticlinal

In plants, a type of cell division where the plane of division is near a right angle to the outer surface of the plant part.

Anticoagulant

A substance that inhibits the clotting of blood.

Anticodon

A certain sequence of nucleotides on tRNA that correspond to codons on a molecule of mRNA. These sequences on both anticodons and codons come in sets of three called "triplets."

Antigen

A substance that triggers the formation of antibodies.

Antioxidant

Any substance that slows down the process of oxidation.

Antiport

The transportation of two chemicals across the same membrane at the same time, but in opposite directions.

Antiseptic

Any substance that either kills, or stops the growth of harmful microbes.

Apetalous

Image of a wood anemone, Anemone quinqefolia.

Flowers without petals.

The flower in the image to the left is called a wood anemone, or Anemone quinquefolia. Image can be found at Spring Wildflower Hike.

Apical bud

The growing end at the tip of a plant.

Apical Complex

A set of anterior structures that characterise one or several developmental stages of members of the protozoan phylum Apicomplexa; includes the following structures, visible by electron microscopy: polar ring, conoid, rhoptries, micronemes, and subpellicular tubules.

Apical Dominance

The influence of the apical bud that inhibits the growth of other buds.

Apical meristem

The region of a root or a stem at which the tip (apex) is growing.

Aplanogamete

A stationary gamete that is found in many seed plants.

Aplanospore

A stationary asexual spore found in non-seed bearing plants.

Apoenzyme

The portion of an enzyme that consists of an inactive protein.

Apogamous

A type of reproduction where a sporophyte forms directly from a gametophyte, without a sexual fusion of gametes. This often happens in ferns.

Apogeotropic

In plants, when roots grow away from the earth.

Apomixis

Any type of reproduction that doesn't involve fertilization or meiosis.

Apoptosis

Programmed cell death.

Apothecium

A cup-like fruit body found on some fungi.

Apparent magnitude

A measure of brightness of an object relative to the observer. Astronomers denote apparent magnitude with the symbol m.

Arcsecond

A unit of measurement that is equal to 1/3600° (In relation to a circle, which has 360°.)

Artery

In animals, a vascular tissue that carries blood from the heart to the rest of the body.

Arthropod

Any member of the phylum Arthropoda, which includes insects, spiders, and crabs. Arthropods are characterized by a chitinous covering, also known as an exoskeleton, and jointed appendages.

Ascus

A microscopic, sac-like structure that contains spores for an order of fungi called the Ascomycetes. Plural "asci."

Asexual

Literally "without sex" or "gender." Used to describe organisms without the capability of reproducing by sex.

Asteroid

General name given to any of a wide variety of relatively small, rocky objects in space, specifically in our solar system. They can range in size from 10 meters to around 1000 km. Most of the asteroids in our solar sytem can be found in orbit between Mars and Jupiter.

Autotroph

An organism that uses carbon dioxide (CO₂) as its principal carbon source.

Axial symmetry

Symmetry about a common axis.

Axoneme

An organelle consisting of a bundle of microtubules extending out of a cell and used for locomotion. This bundle comes in the form of 9 paired outer microtubules surrounding an inner pair. These make up individual undulipodia.

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