Definition of cantus firmus in English:
 cantus firmus
nounPlural cantus firmiˌkantəs ˈfəːməsˌkan(t)əs ˈfərməs
Music A melody used as the basis for a polyphonic composition.
 this four-part mass uses as its cantus firmus a popular French tune
 Example sentencesExamples
-  This cadence articulates the structure of the cantus firmus more clearly than the first, marking precisely the end of the second line of the hymn.
 -  In the early years of the seventeenth century, English composers increasingly turned to the hexachord as a cantus firmus for keyboard pieces.
 -  Orgelbuchlein Pieces - Bach's ‘Little Organ Book’ consists of 45 short chorales mainly having the cantus firmus in the soprano voice with the lower voices acting as counterpoint to the chorale melodies.
 -  On the other hand, the In nomine was still alive in Facy's day, and Lugge was composing his cantus firmus settings, so the genre was not quite dead.
 -  The cantus firmus is sounded in semibreves in the middle of the three voices.
 
Origin
  
Mid 19th century: from Latin, literally 'firm song'.