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verb (used without object)
  1. to become decomposed; rot: vegetation that was decaying.
  2. to decline in excellence, prosperity, health, etc.; deteriorate.
  3. 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)
  1. to cause to decay or decompose; rot: The dampness of the climate decayed the books.
  1. decomposition; rot: Decay made the wood unsuitable for use.
  2. a gradual falling into an inferior condition; progressive decline: the decay of international relations; the decay of the Aztec civilizations.
  3. decline in or loss of strength, health, intellect, etc.: His mental decay is distressing.
  4. 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.
  5. Aerospace. the progressive, accelerating reduction in orbital parameters, particularly apogee and perigee, of a spacecraft due to atmospheric drag.

Origin of decay

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.
Related formsde·cay·a·ble, adjectivede·cayed·ness [dih-keyd-nis, -key-id-] /dɪˈkeɪd nɪs, -ˈkeɪ ɪd-/, adjectivede·cay·less, adjectivenon·de·cayed, adjectivenon·de·cay·ing, adjectivere·de·cay, verbsem·i·de·cay, nounsem·i·de·cayed, adjectiveun·de·cay·a·ble, adjectiveun·de·cayed, adjectiveun·de·cay·ing, adjective

Synonyms for decay

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Synonym study

1. 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. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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Contemporary Examples of decay

Historical Examples of decay

British Dictionary definitions for decay


  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. Also: disintegrate (intr) physics
    1. (of an atomic nucleus) to undergo radioactive disintegration
    2. (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
  1. the process of decline, as in health, mentality, beauty, etc
  2. the state brought about by this process
  3. decomposition, as of vegetable matter
  4. rotten or decayed matterthe dentist drilled out the decay
  5. physics
    1. See radioactive decay
    2. a spontaneous transformation of an elementary particle into two or more different particles
    3. of an excited atom or molecule, losing energy by the spontaneous emission of photons
  6. physics a gradual decrease of a stored charge, magnetic flux, current, etc, when the source of energy has been removedSee also time constant
  7. music the fading away of a note
Derived Formsdecayable, adjective

Word Origin for decay

C15: from Old Northern French decaïr, from Late Latin dēcadere, literally: to fall away, from Latin cadere to fall
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for decay

late 15c., "to decrease," from Anglo-French decair, Old North French decair (Old French decheoir, 12c., Modern French déchoir) "to fall, set (of the sun), weaken, decline, decay," from Vulgar Latin *decadere "to fall off," from de- (see de-) + Latin cadere "to fall" (see case (n.1)). Meaning "decline, deteriorate" is c.1500; that of "to decompose, rot" is from 1570s. Related: Decayed; decaying.


mid-15c., "deterioration, decline in value," from decay (v.). Meaning "gradual decrease in radioactivity" is from 1897.

Online Etymology Dictionary, © 2010 Douglas Harper

decay in Medicine


  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.
  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.

decay in Science


  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.
  1. To undergo decay.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.