Aleatoric/Chance Music

music in which some element of the composition is left to chance, and/or some primary element of a composed work's realization is left to the determination of its performer(s). The term is most often associated with procedures in which the chance element involves a relatively limited number of possibilities. According to acoustician, Werner Meyer-Eppler, a process is said to be aleatoric ... if its course is determined in general but depends on chance in detail.

Other Terms & Sub-Genres: Indeterminate Music, Open Form, Aleatoricism, Algorithmic Music, Generative Music, Stochastic Music

Source: http://en.wikipedia.org/wiki/Aleatoric_music