dialogue (n.)

early 13c., "literary work consisting of a conversation between two or more persons," from Old French dialoge, from Latin dialogus, from Greek dialogos "conversation, dialogue," related to dialogesthai "converse," from dia- "across" (see dia-) + legein "speak," from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')."

Sense broadened to "a conversation" c. 1400. Mistaken belief that it can only mean "conversation between two persons" is from confusion of dia- and di- (1); the error goes back to at least 1532, when trialogue was coined needlessly for "a conversation between three persons." A word that has been used for "conversation between two persons" is the hybrid duologue (1864).