Quiz: Remember the definition of mal de mer?
late 13c., the name of various musical instruments, from Old French symphonie "harmony" (12c.), from Latin symphonia "a unison of sounds, harmony," from Greek symphonia "harmony, concert," from symphonos "harmonious," from syn- "together" (see syn-) + phone "voice, sound" (see fame (n.)).
Meaning "harmony of sounds" is attested from mid-15c.; sense of "music in parts" is from 1590s. "It was only after the advent of Haydn that this word began to mean a sonata for full orchestra. Before that time it meant a prelude, postlude, or interlude, or any short instrumental work." ["Elson's Music Dictionary"] Meaning "elaborate orchestral composition" first attested 1789 (symphonic in this sense is from 1864). Elliptical for "symphony orchestra" from 1926.