plainsong
Concise Oxford English Dictionary © 2008 Oxford University Press:
plainsong/ˈpleɪnsɒŋ/
▶noun unaccompanied church music sung in unison in medieval modes and in free rhythm corresponding to the accentuation of the words, which are taken from the liturgy.
– origin ME: translating L. cantus planus.
'plainsong' also found in these Oxford entries:

