harmonium n.

late 14c., "combination of tones pleasing to the ear," from Old French harmonie, 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; joint, shoulder," from PIE ar(ə)-smo-, suffixed form of root *ar- "to fit together." Modern scientific harmony, using combinations of notes to form chords, is from 16c. Sense of "agreement of feeling, concord" is from late 14c.