1640s, from heterogeneous + -ity, or else from Medieval Latin heterogeneitas, from heterogeneus.
Entries linking to heterogeneity
heterogeneous adj.
"diverse in kind or nature," 1620s, from Medieval Latin heterogeneus, from Greek heterogenes, from heteros "different" (see hetero-) + genos "kind, gender, race stock" (from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups). Earlier in same sense was heterogeneal (c. 1600). Related: Heterogeneously; heterogeneousness.
-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]