Merriam-Webster Collegiate Dictionary - decay

 I. verb  Etymology: Middle English, from Anglo-French decair, from Late Latin decadere to fall, sink, from Latin de- + cadere to fall — more at chance  Date: 15th century  intransitive verb  1. to decline from a sound or prosperous condition  2. to decrease usually gradually in size, quantity, activity, or force  3. to fall into ruin  4. to decline in health, strength, or vigor  5. to undergo decomposition ~ing fruit  transitive verb  1. obsolete to cause to ~ ; impair infirmity that ~s the wise — Shakespeare  2. to destroy by decomposition  • ~er noun Synonyms:  ~, decompose, rot, putrefy, spoil mean to undergo destructive dissolution. ~ implies a slow change from a state of soundness or perfection a ~ing mansion. decompose stresses a breaking down by chemical change and when applied to organic matter a corruption the strong odor of decomposing vegetation. rot is a close synonym of decompose and often connotes foulness fruit was left to rot in warehouses. putrefy implies the rotting of animal matter and offensiveness to sight and smell corpses putrefying on the battlefield. spoil applies chiefly to the decomposition of foods keep the ham from spoiling.  II. noun  Date: 15th century  1. gradual decline in strength, soundness, or prosperity or in degree of excellence or perfection  2. a wasting or wearing away ; ruin  3. obsolete destruction, death  4.  a. rot; specifically aerobic decomposition of proteins chiefly by bacteria  b. the product of ~  5. a decline in health or vigor  6. decrease in quantity, activity, or force: as  a. spontaneous decrease in the number of radioactive atoms in radioactive material  b. spontaneous disintegration (as of an atom or a particle)