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.