Indeterministic music, also called "aleatory music", is music based on chance. The composer's creative input to the outcome of a piece is either greatly reduced or even removed completely. "Creative input" includes melody, harmony, instrumentation, etc... even logical arrangement. Theoretically, indeterminism is the opposite of serialism, where the composer controls every minute detail of a piece through numerical patterns.
Far and wide, John Cage is considered the father of indeterminism. He was influenced by the study of Zen philosophy: he determined that a quiet mind was one free of dislikes; but, since dislikes require likes, it must be free of both likes and dislikes. He once said, "You can become narrow minded, literally, by only liking certain things and disliking others, but you can become open-minded, literally, by giving up your likes and dislikes and becoming interested in things." To escape being influenced by his own likes and dislikes, Cage then sought to remove the creative process from his composition, often relying on coin flips and dice rolls to decide where or how to place a certain note.
For all intents and purposes, indeterminism is dead. Many
musical scholars now disregard it as a passing fad of the mid-20th
century. It remains to be seen whether future generations will try
to revive it or whether it will fade away forever.