mid-15c., "a pouring out, outflow, effusion," from out- + infinitive of pour (v.). From 1757 as "action of pouring out," probably a re-coinage, originally transferred, of things spiritual; sense of "that which is poured out" (again, usually transferred) is from 1827. A verb, outpour "to pour forth," is attested from 1670s.
late 14c., diffusioun, "a copious outpouring," from Old French diffusion and directly from Latin diffusionem (nominative diffusio) "a pouring forth," noun of action from past-participle stem of diffundere "scatter, pour out," from dis- "apart, in every direction" (see dis-) + fundere "to pour" (from nasalized form of PIE root *gheu- "to pour"). Meaning "act of diffusing, state of being diffuse" is from 1590s; figurative sense of "a spreading abroad, dispersion" (of knowledge, etc.) is by 1750.
1550s, "act of melting by heat," from French fusion or directly from Latin fusionem (nominative fusio) "an outpouring, effusion," noun of action from fusus, past participle of fundere "to pour, melt" (from nasalized form of PIE root *gheu- "to pour"). 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).