1550s, "act of melting by heat," from Middle French fusion or directly from Latin fusionem (nominative fusio) "an outpouring, effusion," noun of action from fusus, past participle of fundere "to pour, melt" (see found (v.2)). Meaning "union or blending of different things; state of being united or blended" is by 1776; used especially in 19c, of politics, in early 20c. of psychology, atoms, and jazz (in nuclear physics sense, first recorded 1947; in musical sense, by 1972).
