Decay meaning : The process or result of being gradually decomposed. A deterioration of condition; loss of status or fortune. To deteriorate, to get worse, to lose strength or health, to decline in quality. (of organic material) To rot, to go bad. (of an unstable atom) To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons. (of a quantum system) To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon. Loss of airspeed due to drag. To cause to rot or deteriorate.