abiotic Nonliving. Compare biotic. 

acclimation Adjustment to slowly changing new conditions. Compare threshold effect. 

accuracy Extent to which a measurement agrees with the accepted or correct value for that quantity, based on careful measurements by many people. Compare precision.  

acid deposition The falling of acids and acid-forming compounds from the atmosphere to the earth's surface. Acid deposition is commonly known as acid rain, a term that refers only to wet deposition of droplets of acids and acid-forming compounds. Also called acid rain. 

acid solution Any water solution that has more hydrogen ions (H+) than hydroxide ions (OH-); any water solution with a pH less than 7. Also used for acid. Compare basic solution, neutral solution.

active solar heating system System that uses solar collectors to capture energy from the sun and store it as heat for space heating and water heating. Liquid or air pumped through the collectors transfers the captured heat to a storage system such as an insulated water tank or rock bed. Pumps or fans then distribute the stored heat or hot water throughout a dwelling as needed. Compare passive solar heating system.

Adaptation Any genetically controlled structural, physiological, or behavioral characteristic that helps an organism survive and reproduce under a given set of environmental conditions. It usually results from a beneficial mutation. See biological evolution, differential reproduction, mutation, natural selection. Also used for adaptive trait 

adaptive management Flexible management that views attempts to solve problems as experiments, analyzes failures to see what went wrong, and tries to modify and improve an approach before abandoning it. Because of the inherent unpredictability of complex systems, it often uses the precautionary principle as a management tool.See precautionary principle.

adaptive radiation Process in which numerous new species evolve to fill vacant and new ecological niches in changed environments, usually after a mass extinction or mass depletion. Typically, this takes millions of years.

advanced sewage treatment Specialized chemical and physical processes that reduce the amount of specific pollutants left in wastewater after primary and secondary sewage treatment. This type of treatment usually is expensive. Also tertiary sewage treatment. See also primary sewage treatment, secondary sewage treatment. 

aerobic respiration Complex process that occurs in the cells of most living organisms, in which nutrient organic molecules such as glucose (C6H12O6) combine with oxygen (O2) and produce carbon dioxide (CO2), water (H2O), and energy. Compare photosynthesis.

age structure Percentage of the population (or number of people of each sex) at each age level in a population.

agricultural revolution Gradual shift from small, mobile hunting and gathering bands to settled agricultural communities in which people survived by learning how to breed and raise wild animals and to cultivate wild plants near where they lived. It began 10,000-12,000 years ago. Compare environmental revolution, hunter-gatherers, industrial revolution, information and globalization revolution. 

agroforestry Planting trees and crops together. 

air pollution One or more chemicals in high enough concentrations in the air to (1) harm humans, other animals, vegetation, or materials or (2) alter climate. Excess heat and noise are also considered forms of air pollution. Such chemicals or physical conditions are called air pollutants. See primary pollutant, secondary pollutant. albedo Ability of a surface to reflect light. 

allele Slightly different molecular form found in a particular gene. 

alley cropping Planting of crops in strips with rows of trees or shrubs on each side. 

altitude Height above sea level. Compare latitude. 

anaerobic respiration Form of cellular respiration in which some decomposers get the energy they need through the breakdown of glucose (or other nutrients) in the absence of oxygen.

Also used is fermentation. Compare aerobic respiration. 

animal manure Dung and urine of animals used as a form of organic fertilizer. Compare green manure.

Animals Eukaryotic, multicelled organisms usually capable of mobility and unable to produce their own food (heterotrophs), such as sponges, jellyfishes, arthropods (insects, shrimp, lobsters), mollusks (snails, clams, oysters, octopuses), fish, amphibians (frogs, toads, salamanders), reptiles (turtles, lizards, alligators, crocodiles, snakes), birds, and mammals (kangaroos, bats, cats, rabbits, elephants, whales, porpoises, monkeys, apes, humans). See carnivores, herbivores, omnivores. 

annual Plant that grows, sets seed, and dies in one growing season. Compare perennial.

anthropocentric Human-centered. Compare biocentric. 

Aquaculture Growing and harvesting of fish and shellfish for human use in freshwater ponds, irrigation ditches, and lakes, or in cages or fenced-in areas of coastal lagoons and estuaries. See fish farming, fish ranching. 

aquatic Pertaining to water. Compare terrestrial. 

aquatic life zone Marine and freshwater portions of the biosphere. Examples include freshwater life zones (such as lakes and streams) and ocean or marine life zones (such as estuaries, coastlines, coral reefs, and the deep ocean). 

aquifer Porous, water-saturated layers of sand, gravel, or bedrock that can yield an economically significant amount of water. 

arable land Land that can be cultivated to grow crops. 

area strip mining Type of surface mining used where the terrain is flat. An earthmover strips away the overburden, and a power shovel digs a cut to remove the mineral deposit. After removal of the mineral, the trench is filled with overburden, and a new cut is made parallel to the previous one. The process is repeated over the entire site. Compare dredging, mountaintop removal, open-pit mining, subsurface mining. 

arid Dry. A desert or other area with an arid climate that has little precipitation. 

artificial selection Process by which humans select one or more desirable genetic traits in the population of a plant or animal and then use selective breeding to end up with populations of the species containing large numbers of individuals with the desired traits. Compare genetic engineering, natural selection.

asexual reproduction Reproduction in which a mother cell divides to produce two identical daughter cells that are clones of the mother cell. This type of reproduction is common in single-celled organisms. Compare sexual reproduction. 

atmosphere Whole mass of air surrounding the earth. See stratosphere, troposphere. 

Atom Minute unit made of subatomic particles that is the basic building block of all chemical elements and thus all matter; the smallest unit of an element that can exist and still have the unique characteristics of that element. Compare ion, molecule. 

atomic number Number of protons in the nucleus of an atom. Compare mass number.