Source : Webster's Revised Unabridged Dictionary (1913)

Decay \De*cay"\, v. i. [imp. & p. p. {Decayed}; p. pr. & vb. n.
   {Decaying}.] [OF. decaeir, dechaer, decheoir, F. d['e]choir,
   to decline, fall, become less; L. de- + cadere to fall. See
   {Chance}.]
   To pass gradually from a sound, prosperous, or perfect state,
   to one of imperfection, adversity, or dissolution; to waste
   away; to decline; to fail; to become weak, corrupt, or
   disintegrated; to rot; to perish; as, a tree decays; fortunes
   decay; hopes decay.

         Ill fares the land, to hastening ills a prey, Where
         wealth accumulates and men decay.        --Goldsmith.

Decay \De*cay"\, v. t.
   1. To cause to decay; to impair. [R.]

            Infirmity, that decays the wise.      --Shak.

   2. To destroy. [Obs.] --Shak.

Decay \De*cay"\, n.
   1. Gradual failure of health, strength, soundness,
      prosperity, or of any species of excellence or perfection;
      tendency toward dissolution or extinction; corruption;
      rottenness; decline; deterioration; as, the decay of the
      body; the decay of virtue; the decay of the Roman empire;
      a castle in decay.

            Perhaps my God, though he be far before, May turn,
            and take me by the hand, and more - May strengthen
            my decays.                            --Herbert.

            His [Johnson's] failure was not to be ascribed to
            intellectual decay.                   --Macaulay.

            Which has caused the decay of the consonants to
            follow somewhat different laws.       --James Byrne.

   2. Destruction; death. [Obs.] --Spenser.

   3. Cause of decay. [R.]

            He that plots to be the only figure among ciphers,
            is the decay of the whole age.        --Bacon.

   Syn: Decline; consumption. See {Decline}.

Source : WordNet®

decay
     n 1: the process of gradually becoming inferior
     2: a gradual decrease; as of stored charge or current [syn: {decline}]
     3: the organic phenomenon of rotting [syn: {decomposition}]
     4: an inferior state resulting from the process of decaying;
        "the corpse was in an advanced state of decay"; "the house
        had fallen into a serious state of decay and disrepair"
     5: the spontaneous disintegration of a radioactive substance
        along with the emission of ionizing radiation [syn: {radioactive
        decay}, {disintegration}]
     v 1: lose a stored charge, magnetic flux, or current; "the
          particles disintegrated during the nuclear fission
          process" [syn: {disintegrate}, {decompose}]
     2: fall into decay or ruin; "The unoccupied house started to
        decay" [syn: {crumble}, {delapidate}]
     3: undergo decay or decomposition; "The body started to decay
        and needed to be cremated"

Source : Free On-Line Dictionary of Computing

decay
     
        [Nuclear physics] An automatic conversion which is applied to
        most array-valued expressions in {C}; they "decay into"
        pointer-valued expressions pointing to the array's first
        element.  This term is not used in the official standard for
        the language.
     
        [{Jargon File}]