verb (used without object)
to become decomposed; rot: vegetation that was decaying.
to decline in excellence, prosperity, health, etc.; deteriorate.
Physics. (of a radioactive nucleus) to change spontaneously into one or more different nuclei in a process in which atomic particles, as alpha particles, are emitted from the nucleus, electrons are captured or lost, or fission takes place.
verb (used with object)
to cause to decay or decompose; rot: The dampness of the climate decayed the books.
decomposition; rot: Decay made the wood unsuitable for use.
a gradual falling into an inferior condition; progressive decline: the decay of international relations; the decay of the Aztec civilizations.
decline in or loss of strength, health, intellect, etc.: His mental decay is distressing.
Also called disintegration, radioactive decay. Physics. a radioactive process in which a nucleus undergoes spontaneous transformation into one or more different nuclei and simultaneously emits radiation, loses electrons, or undergoes fission.
Aerospace. the progressive, accelerating reduction in orbital parameters, particularly apogee and perigee, of a spacecraft due to atmospheric drag.

1425–75; (v.) late Middle English decayen < Old North French decair, equivalent to de- de- + cair to fall < Vulgar Latin *cadēre, for Latin cadere; (noun) late Middle English, derivative of the v.

decayable, adjective
decayedness [dih-keyd-nis, -key-id-] , adjective
decayless, adjective
nondecayed, adjective
nondecaying, adjective
redecay, verb
semidecay, noun
semidecayed, adjective
undecayable, adjective
undecayed, adjective
undecaying, adjective

1. degenerate, wither; putrefy. Decay, decompose, disintegrate, rot imply a deterioration or falling away from a sound condition. Decay implies either entire or partial deterioration by progressive natural changes: Teeth decay. Decompose suggests the reducing of a substance to its component elements: Moisture makes some chemical compounds decompose. Disintegrate emphasizes the breaking up, going to pieces, or wearing away of anything, so that its original wholeness is impaired: Rocks disintegrate. Rot is a stronger word than decay and is especially applied to decaying vegetable matter, which may or may not emit offensive odors: Potatoes rot. 5. putrefaction. 7. deterioration, decadence, impairment, dilapidation, degeneration. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
decay (dɪˈkeɪ)
1.  to decline or cause to decline gradually in health, prosperity, excellence, etc; deteriorate; waste away
2.  to rot or cause to rot as a result of bacterial, fungal, or chemical action; decompose
3.  (intr) physics Also: disintegrate
 a.  (of an atomic nucleus) to undergo radioactive disintegration
 b.  (of an elementary particle) to transform into two or more different elementary particles
4.  (intr) physics (of a stored charge, magnetic flux, etc) to decrease gradually when the source of energy has been removed
5.  the process of decline, as in health, mentality, beauty, etc
6.  the state brought about by this process
7.  decomposition, as of vegetable matter
8.  rotten or decayed matter: the dentist drilled out the decay
9.  physics
 a.  See radioactive decay
 b.  a spontaneous transformation of an elementary particle into two or more different particles
 c.  of an excited atom or molecule, losing energy by the spontaneous emission of photons
10.  physics See also time constant a gradual decrease of a stored charge, magnetic flux, current, etc, when the source of energy has been removed
11.  music the fading away of a note
[C15: from Old Northern French decaïr, from Late Latin dēcadere, literally: to fall away, from Latin cadere to fall]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1460, from O.Fr. decair, from V.L. *decadere "to fall off," from L. cadere "to fall" (see case (1)). Meaning "gradual decrease in radioactivity" is from 1897.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

decay de·cay (dĭ-kā')

  1. The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot.

  2. Dental caries.

  3. The loss of information that was registered by the senses and processed into the short-term memory system.

  4. Radioactive decay.

v. de·cayed, de·cay·ing, de·cays
  1. To break down into component parts; rot.

  2. To disintegrate or diminish by radioactive decay.

  3. To decline in health or vigor; waste away.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
decay  [%PREMIUM_LINK%]     (dĭ-kā')  Pronunciation Key 
  1. The breaking down or rotting of organic matter through the action of bacteria, fungi, or other organisms; decomposition.

  2. The spontaneous transformation of a relatively unstable particle into a set of new particles. For example, a pion decays spontaneously into a muon and an antineutrino. The decay of heavy or unstable atomic nuclei (such as uranium or carbon-10) into more stable nuclei and emitted particles is called radioactive decay. The study of particle decay is fundamental to subatomic physics. See more at fundamental force, radioactive decay.

Verb   To undergo decay.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Some poles were taken to museums, while others decayed or were destroyed.
Of course, they may have used sophisticated wood or bamboo tools that have
Over time these wooden posts decayed, leaving deep indentations in a circular
The actual tissue has decayed over the millennia, and has been replaced by
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