"word having the same sense as another," early 15c. (but usually in plural form before 18c., or, if singular, as synonyma), from Old French synonyme (12c.) and directly from Late Latin synonymum, from Greek synonymon "word having the same sense as another," noun use of neuter of synonymos "having the same name as, synonymous," from syn- "together, same" (see syn-) + onyma, Aeolic dialectal form of onoma "name" (from PIE root *no-men- "name").
-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]