From Longman Dictionary of Contemporary Englishplainsongplain‧song /ˈpleɪnsɒŋ $ -sɒːŋ/ (also plainchant) noun [uncountable] APMa type of old Christian church music in which people sing a simple tune without musical instruments
Examples from the Corpus
plainsong• Most chanting is unaccompanied, often because no one is available with the required skill, especially to accompany plainsong.• They gather only for services of prayer and plainsong. 