similarity vs similar

similarity is a noun but similar is not a noun.

Word	Noun	Adjective	Verb	Adverb
similarity	Yes	No	No	No
similar	No	Yes	No	No

similarity (noun)	similar (noun)
the quality of being similar
a Gestalt principle of organization holding that (other things being equal) parts of a stimulus field that are similar to each other tend to be perceived as belonging together as a unit

similarity (adjective)	similar (adjective)
	marked by correspondence or resemblance
	resembling or similar; having the same or some of the same characteristics; often used in combination
	having the same or similar characteristics
	capable of replacing or changing places with something else; permitting mutual substitution without loss of function or suitability
	(of words) expressing closely related meanings