Deriving from the Latin alea, a dice game, "aleatory" refers to any artwork whose composition or performance is partially or wholly determined through chance. This may apply to spoken word (improv standup, off-the-cuff ranting, poetry slams), and theater, and visual art, especially with regard to found-art constructions and collages. (see Hans Arp.) Usually, though, the term is used in a musical context.

In the most common sort of aleatory music, players simply expand and alter the original patterns and sounds suggested by the composer. Improv. Jazz.

More complicated to explain and implement is the type of aleatory music in which the player depends on unpredictable events to guide the whole of his composition or performance. This sort of thing started catching on around 1945, begun by composers who differed widely as to the methods by which they relied on random selection. Notes, pitch, volume, all aspects of the music may be determined by dice throwing, card drawing, mathematical laws of chance, or interpretations of abstract designs (see also John Cage, we all knew that was coming).

The first well-known example of 20th-century aleatory composition was Cage's Music of Changes, 1951. Other notable musical artists in this field are Pierre Boulez and Innias Xenakis.

Aleatory music is also called "chance music." It is good when it works, and heinous when it doesn't.


thanks to:

http://www.xrefer.com/entry/343450
http://www.eyeneer.com/CCM/Clasdict/aleatory.html
http://www.bartleby.com/65/al/aleatory.html