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late 14c., from Old French armonie "harmony," also the name of a musical instrument (12c.), from Latin harmonia, from Greek harmonia "agreement, concord of sounds," also as a proper name, the personification of music, literally "means of joining," used of ship-planks, etc., also "settled government, order," related to harmos "fastenings of a door; shoulder," from PIE *ar-ti-, from *ar- "to fit together" (see arm (n.1)). Musical sense is oldest in English; that of "agreement of feeling, concord" is from late 14c.
The sounding of two or more musical notes at the same time in a way that is pleasant or desired. Harmony, melody, and rhythm are elements of music.