decay

Etymology

From Middle English decayen, dekeyen (“to decrease, diminish”), from Anglo-Norman decaeir (“to fall away, decay, decline”), from Vulgar Latin *dēcadere, etymologically restored form of Latin dēcidere (“to fall away, fail, sink, perish”), from de (“down”) + cadere (“to fall”). Compare decadent and decadence.

noun

1. The process or result of being gradually decomposed.
   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. 1895, H. G. Wells, chapter X, in The Time Machine
2. A deterioration of condition; loss of status or fortune.
3. (obsolete) Overthrow, downfall, ruin.
4. (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

1. (intransitive) To deteriorate, to get worse, to lose strength or health, to decline in quality.
   1. (intransitive, electronics, of storage media or the data on them) To undergo bit rot, that is, gradual degradation.
   2. (intransitive, computing, of software) To undergo software rot, that is, to fail to be updated in a changing environment, so as to eventually become legacy or obsolete.
   3. (intransitive, physics, of a satellite's orbit) To undergo prolonged reduction in altitude (above the orbited body).
      Damaged on lift-off, Skylab was left in orbit until its orbit decayed. 2009, Francis Lyall, Paul B. Larsen, Space Law: A Treatise, page 120
2. (intransitive, of organic material) To rot, to go bad.
   The cat's body decayed rapidly.
3. (intransitive, transitive, physics, chemistry, of an unstable atom) To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons; to undergo radioactive decay.
   Uranium decays to radium through a long series of steps with a cumulative half-life of 4.4 billion years. 2005, Encyclopedia of Earth Science, edited by Timothy M. Kusky, page 349
4. (intransitive, transitive, physics, of a quantum system) To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon.
5. (intransitive, aviation) Loss of airspeed due to drag.
6. (transitive) To cause to rot or deteriorate.
   The extreme humidity decayed the wooden sculptures in the museum's collection in a matter of years.
7. (programming, intransitive) Of an array: to lose its type and dimensions and be reduced to a pointer, for example when passed to a function.