Webster's English Dictionary

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decay

1. de.cay \di-'ka-\ vb [ME decayen, fr. ONF decai:r, fr. LL decadere to 
   fall, sink,] fr. L de- + cadere to fall 1: to decline from a sound or 
   prosperous condition  2: to decrease gradually in quantity, activity, or 
   force  3: to fall into ruin  4: to decline in health, strength, or vigor  
   5: to undergo decomposition  obs  1: to cause to decay : IMPAIR  2: to 
   destroy by decomposition mean to undergo destructive dissolution. DECAY 
   implies a slow change from a state of soundness or perfection; DECOMPOSE 
   stresses a breaking down by chemical change and when applied to organic 
   matter a corruption; ROT is a close synonym of DECOMPOSE and often connotes 
   foulness; PUTREFY implies the rotting of animal matter and offensiveness to 
   sight and smell; SPOIL applies chiefly to the decomposition of foods SYN 
   syn DECAY, DECOMPOSE, ROT, PUTREFY, SPOIL 
2. decay n 1: gradual decline in strength, soundness, or prosperity or in 
   degree of ex cellence or perfection 2: a wasting or wearing away : RUIN  
   obs  3: DESTRUCTION, DEATH  4a: ROT; specif : aerobic decomposition of 
   proteins chiefly b y bacteria 4b: the product of decay  5: a decline in 
   health or vigor  6: decrease in quantity, activity, or force : as  6a: 
   spontaneous decrease in the number of radioactive atoms in radioactive  
   material 6b: spontaneous disintegration (as of an atom or a meson)