insularity (n.)

1610s, "of or pertaining to an island," from Late Latin insularis "of or belonging to an island," from Latin insula "island" (see isle). Metaphoric sense "narrow, prejudiced" is from 1775, from notion of being isolated and cut off from intercourse with other nations or people (an image that naturally suggested itself in Great Britain). The earlier adjective in the literal sense was insulan (mid-15c.), from Latin insulanus.

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]