Contrafact

In jazz, a contrafact is a musical composition consisting of a new melody overlaid on a familiar harmonic structure. Contrafact can also be explained as the use of borrowed chord progressions.

As a compositional device, it was of particular importance in the 1940s development of bop, since it allowed jazz musicians to create new pieces for performance and recording on which they could immediately improvise, without having to seek permission or pay publisher fees for copyrighted materials (while melodies can be copyrighted, the underlying harmonic structure cannot be).