Any music that challenges the commonly accepted notions of what music is and overlaps with avant garde music.

John Cage was a pioneer in experimental music and defined and gave credibility to the form. The term is often used to distinguish the work of American modernist composers (such as John Cage, Christian Wolff, Earle Brown, Meredith Monk, Malcolm Goldstein, Morton Feldman, Terry Riley, La Monte Young, Philip Glass, Steve Reich, etc.) from the European avant-garde at the time (e.g. Stockhausen, Boulez). The 'experiment' in this case is not whether a piece succeeds or fails, but is in the fact that the outcome of the piece is uncertain or unforeseeable (John Cage).

The term was also used contemporaneously for electronic music, particularly in the early musique concrète work of Schaeffer and Henry in France.