Pronunciation: (plān'sông", -song"),[key] —n. 1. the unisonous vocal music used in the Christian church from the earliest times.
2. modal liturgical music; Gregorian chant.
3. a cantus firmus or theme chosen for contrapuntal development.
4. any simple and unadorned melody or air. Also,plain' song".Also called plainchant(for defs. 1, 2).