Music developed for the unaccompanied unison singing of Christian liturgies, based on a system of modes (see mode, n. 1c) and performed in free…

= plainsong, n. 1.

Of or pertaining to Pope Gregory I (who reigned 590–600); chiefly applied to the ancient system of ritual music, otherwise known as plainchant, n. or…

= plainsong, n.

‘The simple unadorned melody of the ancient hymns and chants of the church’ (Grove); plain-song.

A Gregorian chant.

(see quot.).