late 15c., from Latin interior "inner, interior, middle," comparative adjective of inter "within" (from PIE *enter "between, among," comparative of root *en "in"). Specific meaning "away from the coast, of the interior parts of a country" is from 1777. Interior decoration first attested 1769; interior decorator is from 1830. Interior design from 1927.
-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]