Etymology
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decay (v.)

late 15c., "to decrease," also "to decline, deteriorate, lose strength or excellence," 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 "off" (see de-) + Latin cadere "to fall" (from PIE root *kad- "to fall").

Transitive sense of "cause to deteriorate, cause to become unsound or impaired" is from 1530s. Sense of "decompose, rot" is from 1570s. Related: Decayed; decaying.

decay (n.)

mid-15c., "deterioration, decline in value, gradual loss of soundness or perfection," from decay (v.). Obsolete or archaic in reference to fortune or property; meaning "decomposition of organic tissue" is from 1590s. In physics, the meaning "gradual decrease in radioactivity" is by 1897.

updated on July 15, 2018

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Definitions of decay from WordNet
1
decay (n.)
the process of gradually becoming inferior;
decay (n.)
a gradual decrease; as of stored charge or current;
Synonyms: decline
decay (n.)
the organic phenomenon of rotting;
Synonyms: decomposition
decay (n.)
an inferior state resulting from the process of decaying;
the corpse was in an advanced state of decay
the house had fallen into a serious state of decay and disrepair
decay (n.)
the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation;
Synonyms: radioactive decay / disintegration
2
decay (v.)
lose a stored charge, magnetic flux, or current;
Synonyms: disintegrate / decompose
decay (v.)
fall into decay or ruin;
The unoccupied house started to decay
Synonyms: crumble / dilapidate
decay (v.)
undergo decay or decomposition;
The body started to decay and needed to be cremated
From wordnet.princeton.edu, not affiliated with etymonline.