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Very rare or seldom seen; unusual. It often describes natural phenomena, rare diseases, or historical events that happen once in a long time.

To move or swing back and forth at a regular speed; regular fluctuation of a value or quantity above and below a mean value.

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.

To produce offspring; to reproduce. A core vocabulary word for TOEFL passages about life cycles and biology.

To become extinct or to disappear completely so that no trace is left.

The process where material is deposited or settles at the bottom of a liquid; also refers to geological sedimentation.

The height of an object or place above sea level.

The process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll.