IPA: /ˈpleɪnˌsɔŋ/
KK: /ˈplānˌsôŋ/
A type of simple, unaccompanied church music that has a single melodic line and is often used in religious ceremonies.
The choir performed a beautiful plainsong during the mass.
Plainsong is formed from "plain" (from Old French *plain*, meaning flat or level) and "song" (from Old English *sang*, meaning a musical composition). The term refers to a simple, unadorned style of music, typically associated with medieval church music that is straightforward and without harmony.
Think of 'plain' meaning flat or simple, and 'song' as a musical piece — plainsong is a simple, straightforward type of music.