A Rhapsody in music is a one-movement work that is episodic yet integrated, free-flowing in structure, featuring a range of highly contrasted moods, colour and tonality. An air of spontaneous inspiration and a sense of improvisation make it freer in form than a set of variations. Sergei Rachmaninoff's set of variations on a theme by Niccolò Paganini are so free in structure that the composer called them a Rhapsody on a Theme of Paganini.   Wikipedia