equivalence (n.)

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.

"simultaneous conflicting feelings," 1924 (by 1912 as ambivalency), from German Ambivalenz, coined 1910 by Swiss psychologist Eugen Bleuler on the model of German Equivalenz "equivalence," etc. (for which see equivalence), from Latin ambi- "both, on both sides" (see ambi-) + valentia "strength," an abstract noun from the present participle of valere "be strong" (from PIE root *wal- "to be strong"). A psychological term that by 1929 had taken on a broader literary and general sense.