Pronunciation: (di-kā'),[key] —v.i. 1. to become decomposed; rot: vegetation that was decaying. 2. to decline in excellence, prosperity, health, etc.; deteriorate.
3. Physics.(of a radioactive nucleus) to change spontaneously into one or more different nuclei in a process in which atomic particles, as alpha particles, are emitted from the nucleus, electrons are captured or lost, or fission takes place.
—v.t. to cause to decay or decompose; rot: The dampness of the climate decayed the books.
—n. 1. decomposition; rot: Decay made the wood unsuitable for use. 2. a gradual falling into an inferior condition; progressive decline: the decay of international relations; the decay of the Aztec civilizations. 3. decline in or loss of strength, health, intellect, etc.: His mental decay is distressing. 4. Also called disintegration, radioactive decay.Physics.a radioactive process in which a nucleus undergoes spontaneous transformation into one or more different nuclei and simultaneously emits radiation, loses electrons, or undergoes fission.
5. Aerospace.the progressive, accelerating reduction in orbital parameters, particularly apogee and perigee, of a spacecraft due to atmospheric drag.