early 15c., "equal in value, power, or effect," from Late Latin aequivalentem (nominative aequivalens) "equivalent," present participle of aequivalere "be equivalent," from Latin aequus "equal" (see equal (adj.)) + valere "be well, be worth" (from PIE root *wal- "to be strong"). As a noun from c. 1500, "that which is equal or corresponds to." Related: Equivalently.
"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).
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."
"a lady," 1660s, from the common title of respect for Italian and Portuguese ladies, equivalent to Spanish doña, prefixed to the Christian name. The fem. equivalent of don (n.).