Douglas Harper's Etymology Dictionary
also plainsong, plain-song, 1510s, translating Latin cantus planus, French plain chant.
Wiktionary
alt. 1 (context music English) The unisonous vocal music which has been used in the Christian church from its earliest centuries. 2 (context music English) A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song. 3 (context music English) The simple notes of an air, without ornament or variation; hence, a plain, unexaggerated statement. n. 1 (context music English) The unisonous vocal music which has been used in the Christian church from its earliest centuries. 2 (context music English) A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song. 3 (context music English) The simple notes of an air, without ornament or variation; hence, a plain, unexaggerated statement.
Usage examples of "plain-song".
Upon the stage then stepped the figures of this pair of human beings, chanting their ancient plain-song of incantation in the moonlit desert, and working their rites of unholy evocation as the priests had worked them centuries before in the sands that now buried Sakkara fathoms deep.
We go to the eleven o'clock solemn High Mass, with plain-song propers sung by the Ritual Choir (that's Darcy Dwyer's lot) and a missa brevis and motet sung by the Gallery Choir, which is like angels, if angels can sing, which I suppose they do.