Definition and background:
Music in which the composer introduces the elements of chance or unpredictability with regard to either the composition or its performance. This is not a 20th century invention as it was known in the 18th century in the form of dice music in which dice were used to determine which measures of the music would be performed.
For more information on the music term "Aleatory music" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary