noun

• (ecology, medicine) The process or result of being gradually decomposed; rot, decomposition.
  "I fancied at first the stuff was paraffin wax, and smashed the jar accordingly. But the odor of camphor was unmistakable. It struck me as singularly odd, that among the universal decay, this volatile substance had chanced to survive, perhaps through many thousand years."
• A deterioration of condition; loss of status, quality, strength, or fortune.
• (obsolete) Overthrow, downfall, destruction, ruin.
  "Let’s cheere our ſouldiers to incounter him, That grieuous image of ingratitude: That fiery thirſter after Soueraigntie: […] Reſolue my Lords and louing ſouldiers now, To ſaue your king and country from decay:"
• (programming) The situation, in programming languages such as C, where an array loses its type and dimensions and is reduced to a pointer, for example by passing it to a function.

verb

• (intransitive) To deteriorate, to get worse, to lose strength or health, to decline in quality.
  "The pair loved to take pictures in the decaying hospital on forty-third street."
• (intransitive) To deteriorate, to get worse, to lose strength or health, to decline in quality.
• (intransitive) To deteriorate, to get worse, to lose strength or health, to decline in quality.
• (intransitive) To deteriorate, to get worse, to lose strength or health, to decline in quality.
  "Damaged on lift-off, Skylab was left in orbit until its orbit decayed."