1610s, as a term in logic, from Medieval Latin identicus "the same," from Late Latin identitas "identity, sameness," ultimately from comb. form of Latin idem "the same" (from id "it, that one;" see id) + demonstrative suffix -dem. General sense of "being the same or very similar" is from 1630s. Replaced Middle English idemptical (late 15c.), from Medieval Latin idemptitas "identity," from Latin idem. Related: Identically.
nonidentical non·i·den·ti·cal (nŏn'ī-děn'tĭ-kəl)
Not being the same; different.
Fraternal, as of twins.
identical i·den·ti·cal (ī-děn'tĭ-kəl)
Exactly equal and alike.
Of or relating to a twin or twins developed from the same fertilized ovum and having the same genetic makeup and closely similar appearance; monozygotic.