simultaneity (n.)

"existing, occurring, or appearing at the same time," 1650s, from Medieval Latin simultaneus, ultimately from Latin simul "at the same time," which is related to similis "like, resembling, of the same kind" (see similar).

The Medieval Latin word is perhaps via the extended form simultim "at the same time," or from simul with ending abstracted from Late Latin spontaneus, where the -t- is organic. Related: Simultaneously. "Mark Twain" back-formed from it a verb simultane "do (something) at the same time as (something else)," 1880.

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]