Cantus firmus

In music, a cantus firmus ("fixed song") is a pre-existing melody forming the basis of a polyphonic composition.

The plural of this Latin term is cantus firmi, although the corrupt form canti firmi (resulting from the grammatically incorrect treatment of cantus as a second- rather than a fourth-declension noun) can also be found. The Italian is often used instead: canto fermo (and the plural in Italian is canti fermi).