"state, quality, or condition of being similar," 1660s, from similar + -ity, or from French similarité. Related: Similarities "points of resemblance" (1838).
Entries linking to similarity
similar adj.
"having characteristics in common," 1610s (earlier similary, 1560s), from French similaire, from a Medieval Latin extended form of Latin similis "like, resembling, of the same kind," from Old Latin semol "together" (from PIE root *sem- (1) "one; as one, together with"). The noun meaning "that which is similar" is from 1650s. Related: Similarly.
-ity
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]