plainsong vs plainchant

plainsong and plainchant both are nouns.

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

As nouns, plainsong and plainchant are synonyms defined as:

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

plainsong (noun)	plainchant (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