"occurring at the same time," 1775, shortening of synchronical (1650s), from Late Latin synchronus "simultaneous" (see synchronous). Linguistic sense is first recorded 1922, probably a borrowing from French synchronique (de Saussure, 1913). Synchronal "simultaneous" is from 1650s. Related: synchronically.
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]
1580s, "quality of being synchronous," from Modern Latin synchronismus, from Greek synkhronismos, from synkhronos (see synchronous). Meaning "recurring at the same successive instants of time" is from 1854.
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Definitions of synchronicity from WordNet
the relation that exists when things occur at the same time;