S/N vs ratio

S/N and ratio both are nouns.

Word	Noun	Adjective	Verb	Adverb
S/N	Yes	No	No	No
ratio	Yes	No	No	No

As nouns, ratio is a hypernym of S/N; that is, ratio is a word with a broader meaning than S/N:

S/N: the ratio of signal intensity to noise intensity
ratio: the relative magnitudes of two quantities (usually expressed as a quotient)

S/N (noun)	ratio (noun)
the ratio of signal intensity to noise intensity	the relation between things (or parts of things) with respect to their comparative quantity, magnitude, or degree
	the relative magnitudes of two quantities (usually expressed as a quotient)