(Lat.: ‘pre-existing melody’).
The term seems to have been first used in the 13th century by Franco of Cologne in his treatise Ars cantus mensurabilis (see StrunkSR1, 153). As Franco used the term it referred to the pre-existing melody taken as the basis for two-part polyphony, or discantus, and used as the tenor to which a discant voice was added. In later writings the term became generally synonymous with Cantus firmus when that term was used to mean not merely a melody in long note values of central importance to a polyphonic texture but was specifically a pre-existing melody borrowed for a new composition.
LEWIS LOCKWOOD