c. 1300, a name given to various types of musical instruments, from Old French simphonie, sifonie, simfone "musical harmony; stringed instrument" (12c., Modern French symphonie) and directly from Latin symphonia "a unison of sounds, harmony," from Greek symphonia "harmony, concord of sounds," from symphonos "harmonious, agreeing in sound," from assimilated form of syn- "together" (see syn-) + phōnē "voice, sound," from PIE root *bha- (2) "to speak, tell, say."

Meaning "harmony of sounds" in English is attested from late 14c.; 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. Elliptical for "symphony orchestra" from 1926. Diminutive symphonette is recorded from 1947.

1580s, "account of a gathering or party," from Latin symposium "drinking party, symposium," from Greek symposion "drinking party, convivial gathering of the educated" (related to sympotes "drinking companion"), from assimilated form of syn- "together" (see syn-) + posis "a drinking," from a stem of Aeolic ponen "to drink," from PIE root *po(i)- "to drink."

The symposium usually followed a dinner, for the Greeks did not drink at meals. Its enjoyment was heightened by intellectual or agreeable conversation, by the introduction of music or dancers, and by other amusements. [Century Dictionary]

The sense of "a meeting on some subject" is from 1784. Reflecting the Greek fondness for mixing wine and intellectual discussion, the modern sense is especially from the word being used as a title for one of Plato's dialogues. Greek plural is symposia, and the leader of one is a symposiarch (c. 1600 in English). Related: Symposiac (adj.); symposial.

1540s, re-Latinized from sinthoma (late 14c.), from Medieval Latin sinthoma "symptom of a disease," altered from Late Latin symptoma, from Greek symptoma "a happening, accident, disease," from stem of sympiptein "to befall, happen; coincide, fall together," from assimilated form of syn- "together" (see syn-) + piptein "to fall," from PIE *pi-pt-, reduplicated form of root *pet- "to rush; to fly."

Spelling restored in early Modern English in part by influence of Middle French symptome (16c.). General (non-medical) use is from 1610s.

also synesthesia, "sensation in one part of the body produced by stimulus in another," 1881, in some cases via French, from Modern Latin, from Greek syn- "together" (see syn-) + aisthēsis "feeling" (from PIE root *au- "to perceive") + abstract noun ending -ia. Also psychologically, of the senses (colors that seem to the perceiver to having odor, etc.), from 1891. Related: Synaesthetic (adj.).