v. 1
To deteriorate, to get worse, to lose strength or health, to decline in quality.
The pair loved to take pictures in the decaying hospital on forty-third street.
v. 2
To deteriorate, to get worse, to lose strength or health, to decline in quality. To undergo bit rot, that is, gradual degradation.
n. 1
Radioactive decay; decomposition of an atom or its nucleus.
alpha decay—gamma decay—fission decay—decay chain
n. 2
Rot; any processes or result of organic matter being gradually decomposed, especially by microbial action.
tooth decay/dental decay—wood-decay fungi—bacterial decay—photochemical decay

Related words

Word family: decayable deciduous
Phrases: tooth decay gamma decay nuclear decay platform decay radioactive decay bit decay decay chain decay series

Forms

Plural: decays → disintegrations, decompositions, declines, dilapidations…
Past participle: decayed
e.g. decayed → decomposed, disintegrated, rotted, deteriorated…

Choose the right word

Decay, Putrefy, Putrid, Putrescent, Putridity or Putrescence?

Putrefy and the adjectives putrid and putrescent, and the nouns putridity and putrescence, are used almost exclusively of animal matter in a state of decomposition, the more general word decay being used of either animal or vegetable substances.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

rot Put the meat in the refrigerator, or it will rot.
deteriorate Boldi's eyesight began to deteriorate drastically.
disintegrate Comets start to disintegrate as they approach the sun.
decompose Rocks take time to decompose, especially quartz and feldspar.
rust My clunker has more rust than paint at this point.

Popularity over time

Word	1850 → 2019	Trend
decay		falling
rot		falling
deteriorate		steady
disintegrate		steady
decompose		steady
rust		falling

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: rot¹, ebb¹
4 letters: fail¹, sink¹, fade¹, rust¹, wane¹
5 letters: waste¹, spoil¹
6 letters: fading², perish², wither², fester², molder²
7 letters: failure², decline², failing², corrupt², wasting², consume², crumble², dwindle², moulder², mortify³, putrefy
8 letters: dissolve², diminish³, languish², caducity
9 letters: decadence³, decompose³
10 letters: corruption³, degenerate⁴, degeneracy⁵, declension³, dilapidate⁴
11 letters: deteriorate⁵, decrepitude, putrescence
12 letters: degeneration⁵, disintegrate⁴, putrefaction, dilapidation
13 letters: deterioration⁶, decomposition⁵
14 letters: disintegration⁵

The small number after each word is its number of syllables (fading² = 2 syllables).