Cantus Firmus is a musical term that refers to a fixed melody used as the foundation for a polyphonic composition. This melody is often derived from a Gregorian chant or other traditional tunes and serves as a structural anchor around which other musical lines are built. In the context of Renaissance and Baroque music, composers would create intricate harmonies and counterpoints that interact with the cantus firmus. This technique allowed for rich textures and complex musical forms, showcasing the skill of the composer while maintaining a recognizable melodic line.