Cantus firmus refers to the fixed, often Gregorian, melodic line around which polyphonic music is woven. In poetry, it is a metaphor for any pre-existing tradition, form, or theme that the poet takes as a foundation and elaborates. It speaks to the relationship between tradition and innovation, between what is inherited and what is created. The phrase is particularly resonant in discussions of allusion, intertextuality, and the way new poems are always composed in dialogue with older ones.

Cantus firmus literally means firm song or fixed song and refers to the pre-existing melodic line — typically a Gregorian chant melody — around which Renaissance polyphonic composers wove their intricate harmonies and counterpoints. The musical concept became a powerful metaphor for the relationship between tradition and innovation in poetry: the cantus firmus is what is inherited, the counterpoint is what is created. Students should note that T.S. Eliot's concept of tradition in "Tradition and the Individual Talent" is essentially a theory of poetic cantus firmus: the existing monuments of literature form the fixed song against which each new poem must compose its own voice. Related musical and poetic terms include contrapunctus (counterpoint), ostinato (a persistently repeated musical phrase), and the concepts of allusion, intertextuality, and the anxiety of influence as developed by Harold Bloom.