"a seeing or hearing something which is not there," 1640s, from Latin hallucinationem (nominative hallucinatio), earlier alucinatio, noun of action from past-participle stem of alucinari (see hallucinate). Related: Hallucinations.
word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," which is from the same source as genos "birth," genea "race, family," from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. First used in late 18th century French chemistry (see oxygen), it probably involves a misunderstanding of -genes, as though it meant "that which produces."