a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].
early 15c., intervencioun, "intercession, intercessory prayer," Late Latin interventionem (nominative interventio) "an interposing, a giving security," literally "a coming between," noun of action from past-participle stem of Latin intervenire "to come between, interrupt," from inter "between" (see inter-) + venire "to come" (from a suffixed form of PIE root *gwa- "to go, come"). Later "act of intervening" in any way; in 19c.-20c. often of international relations; by 1983 of interpersonal intrusions by friends or family meant to reform a life felt to be going wrong.