Synonyms for Decay:

n.

curdle, better, perish, thrive, go to pot, biodegrade, go bad, fester, taint, go to, go off. breakage, burn, bruise, blemish, chip, burnout, blister. deepen, slip into, go down, descend into, spiral, escalate. devastation, destruction, havoc, demolition, sabotage, harm, the ravages of something, damage. adulteration (noun)
corruption, contamination.
breaking down, collapse (noun)
decadence, downfall, atrophy, disrepair, decomposition, dissolution, depreciation, mortification, rot, adulteration, senescence, consumption, gangrene, disintegration, Crumbling, decline, putrefaction, degeneration, extinction, decrease, failing, putrescence, decrepitude, impairment, deterioration, spoilage, ruination, rust, degeneracy, ruin, blight, dilapidation.
corrosion (noun)
rust.
decay (noun)
decomposition, decompose, disintegrate, decline, disintegration, delapidate, radioactive decay, crumble.
deterioration (noun)
degeneration, degeneracy, dilapidation, decadence, putrefaction, deterioration, retrogression, corruption.
disease (noun)
blight.
disintegration (noun)
dissection, disintegration, gangrene, decomposition, mortification, dissolution, spoilage, chaos.
feebleness (noun)
decrepitude.
impurity (noun)
contamination.
phenomenon (noun)
decomposition.
process (noun)
decline.
ruin (noun)
ruin, havoc.
waste (noun)
effluent, Dejecta, slop, refuse, debris, Filings, residue, detritus, leakage, garbage, chaff, waste, remnant, rubbish, offal, despoilment, junk.

v.

change (verb)
crumble, disintegrate, decompose, dilapidate.
deteriorate (verb)
deteriorate, downgrade, fall apart, dilapidate, revert, atrophy, relapse, tergiversate, sink, slip, fade, moulder, slump, slide, lapse, wane, get worse, wither, go downhill, collapse, regress, ebb, worsen, decline, retrogress, wilt.
deteriorate, crumble (verb)
decompose, fade, disintegrate, mortify, slump, degenerate, go bad, sink, spoil, wither, curdle, perish, turn, mold, putrefy, collapse, wane, mildew, get worse, dissolve.
disintegrate (verb)
crumble, rot, putrefy, decompose, fracture, rupture, anatomize, disintegrate, turn, degenerate, break, mortify, dissolve, spoil, dissect, shatter.

Other synonyms:

devastation, consumption, wear and tear, taint, putrescence, fester, curdle, degradation, carrion, perversion, perish, rottenness, biodegrade, regression. damage, destruction, demolition, breakage, harm, wreckage, breakdown. aggravation, sabotage, mold. ruin, go off. decrepitude
disrepair.
degenerate
go bad.
deteriorate
sink.
die
go bad, dilapidate.
oxidize
rust.
smut
mildew.

Usage examples for decay

  1. Fell into decay in later Muhammadan times. – The Panjab, North-West Frontier Province, and Kashmir by Sir James McCrone Douie
  2. For even the middle- aged, in the decay of their daring, must look for the fresh thought and the fresh impulse to the youth which follows at their heels in the march of life. – The Seaboard Parish, Complete by George MacDonald