Experimental music refers, in the English-language literature, to a compositional tradition which arose in the mid-20th century, applied particularly in North America to music composed in such a way that its outcome is unforeseeable. Its most famous and influential exponent was John Cage.
Some uses of “experimental” describe music that:
[1] Lies within specific genres but pushes against their boundaries and definitions;
[2] Hybridizes disparate musical styles;
[3] Incorporates unorthodox, new, or distinct components;
[4] Incorporates unorthodox, new, or distinct methods (example: musique concrète;
[5] Departs from traditional norms.
(Wikipedia)