suffix forming abstract nouns from adjectives, meaning "condition or quality of being ______," from Middle English -ite, from Old French -ité and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of connective -i- + -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]