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A biological community of interacting organisms and their physical environment. It encompasses the complex relationships between living things and non-living resources.

Lacking in movement, action, or change; in physics, relating to bodies at rest or forces in equilibrium.

To calculate, estimate, or figure out something using data, logic, or mathematical methods. It implies a process of reasoning from known facts to unknown values.

To release, set free, or emit. It can refer to letting a person go, releasing energy in a chemical reaction, or emitting gases.

To settle as sediment; the process of forming a solid in a liquid solution or the accumulation of historical experience.

The throwing back by a body or surface of light, heat, or sound without absorbing it; or a serious thought or consideration.

The branch of biology that deals with the normal functions of living organisms and their parts.

The act of one animal preying on others for food. This is essential vocabulary for TOEFL lectures describing predator-prey relationships.

The state or process of a species, family, or larger group being or becoming extinct. A core concept in TOEFL passages about dinosaurs or environmental conservation.

Any form of water, such as rain, snow, sleet, or hail, that falls to the ground from the atmosphere.