English 'monologue' from Greek 'mónos' (alone) + 'lógos' (speech) — literally 'speaking alone,' modeled on 'dialogue.'
A long speech by one person during a conversation or performance; a dramatic composition for a single performer.
From French monologue (a speech by one person), formed on the model of dialogue by substituting Greek monos (alone, single) for di- (two). Monos traces to PIE *mono- (one, single), cognate with Latin semel (once) and English mono- in compounds. Logos (word, speech, reason) derives from PIE *leg- (to gather, to speak), the same
A 'monologue' is speech by one person; a 'dialogue' is between two (Greek 'dia-' means 'through' or 'across,' not 'two' — the 'two' is coincidental). The Greek 'lógos' (word, reason) may be the most productive root in English: 'biology' (life-word), 'psychology' (soul-word), 'technology' (craft-word), 'logic' (the art of reasoning), 'logo' (a word-symbol), and dozens more '-logy' and '-log' words.