To decay is defined as to rot, lose strength or deteriorate.(verb)
Decay is defined as rotted matter or the state of rotting, deteriorating or declining.(noun)
An example of decay is what has happened to an old abandoned building.
See decay in Webster's New World College Dictionary
Origin: ME decaien < Anglo-Fr & OFr decäir < VL *decadere: see decadence
See decay in American Heritage Dictionary 4
verb de·cayed, de·cay·ing, de·cays verb, intransitive
Origin: Middle English decayen
Origin: , from Old French decair
Origin: , from Vulgar Latin *dēcadere
Origin: : Latin dē-, de-
Origin: + Latin cadere, to fall; see kad- in Indo-European roots.
Learn more about decay