Improvisation is perhaps the central element of jazz. It is at the heart of what makes jazz, even though other musical forms make use of improvisation. But jazz music can also be arranged or orchestrated to frame or complement the improvisation. Improvisation is the impromptu creation of melodies and rhythms and within jazz it allows individual voices of the jazz artist to be heard through its solo elements. The improvising artist takes the musical content and expands its ideas by varying the melody, inventing new melodies, or both. The harmonic structure of the song remains and provides a guide for the improvisation.

Source: National Jazz Archive