Cantus firmus
From Conservapedia
Cantus Firmus is a technique widespread in sacred music from the 10th to the 18th centuries, in which a multi-voice polyphonic work uses a plainchant in one of the voices. The name "cantus firmus," which is Latin for "fixed song," implies that the chant, which would be familiar to the congregation, acts as a slow-moving anchor around which the complexity of the other voices act as decoration.
