decay vs ruin

decay and ruin both are nouns.

Word	Noun	Adjective	Verb	Adverb
decay	Yes	No	Yes	No
ruin	Yes	No	Yes	No

As nouns, ruin is a hyponym of decay; that is, ruin is a word with a more specific, narrower meaning than decay:

As verbs, ruin is a hyponym of decay; that is, ruin is a word with a more specific, narrower meaning than decay:

decay (noun)	ruin (noun)
the organic phenomenon of rotting	destruction achieved by causing something to be wrecked or ruined
a gradual decrease; as of stored charge or current	a ruined building
the process of gradually becoming inferior	an event that results in destruction
the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation	failure that results in a loss of position or reputation
an inferior state resulting from the process of decaying	the process of becoming dilapidated
	an irrecoverable state of devastation and destruction

decay (verb)	ruin (verb)
fall into decay or ruin	fall into ruin
lose a stored charge, magnetic flux, or current	destroy completely; damage irreparably
undergo decay or decomposition	reduce to ruins
	deprive of virginity
	reduce to bankruptcy
	destroy or cause to fail