cantus firmus

noun

cantus fir·​mus -ˈfir-məs 

-ˈfər-

1

: the plainsong or simple Gregorian melody originally sung in unison and prescribed as to form and use by ecclesiastical tradition

2

: a melodic theme or subject

especially : one for contrapuntal treatment

Word History

Etymology

Medieval Latin, literally, fixed song

First Known Use

1761, in the meaning defined at sense 1