v. & n.
v. 1 a intr. rot, decompose. b tr. cause to rot or decompose. 2 intr. & tr. decline or cause to decline in quality, power, wealth, energy, beauty, etc. 3 intr. Physics a (usu. foll. by to) (of a substance etc.) undergo change by radioactivity. b undergo a gradual decrease in magnitude of a physical quantity.
n. 1 a rotten or ruinous state; a process of wasting away. 2 decline in health, quality, etc. 3 Physics a change into another substance etc. by radioactivity. b a decrease in the magnitude of a physical quantity, esp. the intensity of radiation or amplitude of oscillation. 4 decayed tissue. decayable adj.
[ ME f. OF decair f. Rmc (as DE-, L cadere fall) ]