"equality in value, correspondence in signification, force, nature, etc.," 1540s, from French équivalence, from Medieval Latin aequivalentia, from Late Latin aequivalentem "equivalent" (see equivalent). Related: Equivalency (1530s).

word-forming element meaning "both, on both sides," from Latin ambi- "around, round about" (before vowels amb-, also sometimes reduced to am-, an-), from PIE root *ambhi- "around," which is probably an ablative plural (*ant-bhi "from both sides") of *ant- "front, forehead."

Proto-Indo-European root meaning "to be strong."

It forms all or part of: ambivalence; Arnold; avail; bivalent; convalesce; countervail; Donald; equivalent; evaluation; Gerald; Harold; invalid (adj.1) "not strong, infirm;" invalid (adj.2) "of no legal force;" Isold; multivalent; polyvalent; prevalent; prevail; Reynold; Ronald; valediction; valence; Valerie; valetudinarian; valiance; valiant; valid; valor; value; Vladimir; Walter; wield.

It is the hypothetical source of/evidence for its existence is provided by: Latin valere "be strong, be well, be worth;" Old Church Slavonic vlasti "to rule over;" Lithuanian valdyti "to have power;" Celtic *walos- "ruler," Old Irish flaith "dominion," Welsh gallu "to be able;" Old English wealdan "to rule," Old High German -walt, -wald "power" (in personal names), Old Norse valdr "ruler."

"having simultaneous conflicting feelings or contradictory ideas about something," 1916, originally a term in psychology; a back-formation from ambivalence. In general use by 1929.