<p><strong>Verb (used without object)</strong></p><ul><li>To become decomposed; rot: vegetation that was decaying.</li><li>Synonyms: putrefy, wither, degenerate.</li><li>To decline in excellence, prosperity, health, etc.; deteriorate.</li><li>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.</li></ul><p><strong>Verb (used with object)</strong></p><ul><li>To cause to decay or decompose; rot: The dampness of the climate decayed the books.</li></ul><p><strong>Noun</strong></p><ul><li>Decomposition; rot: Decay made the wood unsuitable for use.</li><li>Synonyms: putrefaction.</li><li>A gradual falling into an inferior condition; progressive decline: the decay of international relations; the decay of the Aztec civilizations.</li><li>Decline in or loss of strength, health, intellect, etc.: His mental decay is distressing.</li><li>Synonyms: dilapidation, degeneration, impairment, decadence, deterioration.</li><li>Physics: radioactive decay.</li><li>Aerospace: the progressive, accelerating reduction in orbital parameters, particularly apogee and perigee, of a spacecraft due to atmospheric drag.</li></ul>