plain-song

noun

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.

noun

1. the style of unison unaccompanied vocal music used in the medieval Church, esp in Gregorian chantAlso called: plainchant

n.also plainsong, plain-song, 1510s, translating Latin cantus planus, French plain chant.