plainchant vs plainsong

plainchant and plainsong both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainchant	Yes	No	No	No
plainsong	Yes	No	No	No

As nouns, plainchant and plainsong are synonyms defined as:

plainchant and plainsong: (Roman Catholic Church) a liturgical chant consisting of a single, unaccompanied melodic line

plainchant (noun)	plainsong (noun)
(Roman Catholic Church) a liturgical chant consisting of a single, unaccompanied melodic line	(Roman Catholic Church) a liturgical chant consisting of a single, unaccompanied melodic line