Plainchant (or plainsong) is the generic term for the chant, the ritual (liturgical) music of the Christian church used from the earliest times. The term comes from the Latin “Cantus plainus”, to delineate it from “Cantus figuratus” (with counterpoint) and “Cantus mensuratus” (rhythmic). It developed from the Jewish and Pagan religious music in use at the time and by the 3rd century was in everyday use in services in both the Roman and the various churches in the East when they began to take distinct paths. (Byzantine, Coptic, Slavonic etc.,)
Today it refers mainly to the music of the Western Church – Old Roman, Ambrosian, and especially Gregorian forms which were the only forms of plainchant not suppressed by the Carolingian church. Today, Gregorian chant and plainchant (plainsong) have become synonymous.