Plainchant is an old kind of Christian church singing: one simple tune, usually with no instruments, and often with many voices on the same line rather than harmony. People also call it plainsong; the two names point to the same thing. The most famous kind is Gregorian chant, the Latin singing you hear in old Catholic services — though actually not all plainchant is Gregorian, and other churches have their own plainchant too. The word sits in music history and church talk more than in everyday English. When someone says “plainchant,” they mean this special, spare church song, not just any chant or any “plain” singing.