Definition from Wiktionary, the free dictionary
- (music) The unisonous vocal music which has been used in the Christian church from its earliest centuries.
- (music) A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song.
- (music) The simple notes of an air, without ornament or variation; hence, a plain, unexaggerated statement.
- plain-song in The Century Dictionary, The Century Co., New York, 1911