cantus

Cantus, cantus, m., fourth declension, from cano, canere, cecini, cantum. The fourth declension trap is unavoidable here: cantus is the nominative singular, the genitive singular and the nominative and accusative plural all at once, so only the surrounding words tell you which. The ablative singular cantu is the form the corpus uses most, usually as an ablative of means.

Against carmen, which is the composed thing, the poem or the words, cantus is the singing of it. Medieval Latin narrows it to liturgical chant and builds a vocabulary on top: cantus planus is plainchant, cantus firmus the fixed melody that polyphony is written around, and the man who leads it is the cantor. Classical Latin will also use cantus of an incantation, a spell sung rather than spoken.