conventionality n.

late 15c., "of the nature of an agreement," from Late Latin conventionalis "pertaining to convention or agreement," from Latin conventionem (nominative conventio) "a meeting, assembly; agreement," noun of action from past-participle stem of convenire "unite, be suitable, agree, assemble," from assimilated form of com "with, together" (see con-) + venire "to come" (from PIE root *gwa- "to go, come").

Meaning "of the nature of a formal meeting of delegates" is from 1812, now rare; that of "established by social convention, arising out of custom or usage" is from 1761; sense of "following tradition" is from 1831. Of weapons, "non-nuclear," from 1955. Related: Conventionally.

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]