Which musical element is central to the definition of a cantus firmus?

The definition of a cantus firmus is centered around the concept of a single, pre-existing melody that serves as the foundational element in a musical composition, particularly in polyphonic music of the medieval and Renaissance periods. This melody typically maintains its integrity while being accompanied by other voices or harmonies that interact with it.

In this context, the cantus firmus often serves as the scaffold upon which composers build intricate musical structures. Its role is crucial because it establishes the pitch and some of the rhythmic characteristics for the other voices or instruments that will elaborate on or weave around it, leading to a fuller and more complex musical texture.

The other choices relate to different aspects of music composition, but they do not capture the essence of what a cantus firmus is. A sequence of chords, for instance, involves harmonic relationships rather than a singular melodic line. Similarly, a rhythmic pattern or harmonic progression focuses on rhythm and harmonics, respectively, rather than a distinct melody which defines the cantus firmus. Thus, the identification of a single melody as central to the definition is what establishes its significance in the history of music composition.