Wikipedia
Polystylism is the use of multiple styles or techniques in literature, art, film, or, especially, music, and is a postmodern characteristic.
Some prominent contemporary polystylist composers include Peter Maxwell Davies, Michael Colgrass, Lera Auerbach, Sofia Gubaidulina, George Rochberg, Alfred Schnittke, Django Bates, Alexander Zhurbin, Lev Zhurbin and John Zorn. However, Gubaidulina, among others, has rejected the term as not applicable to her work. Polystylist composers from earlier in the twentieth century include Charles Ives and Eric Satie. Among literary figures, James Joyce has been referred to as a polystylist.
Though perhaps not the original source of the term, the first important discussion of the subject is Alfred Schnittke's essay "Polystylistic Tendencies in Modern Music (1971)". The composers cited by Schnittke as those who make use of polystylism are Alban Berg, Luciano Berio, Pierre Boulez, Edison Denisov, Hans Werner Henze, Mauricio Kagel, Jan Klusák, György Ligeti, Carl Orff, Arvo Pärt, Krzysztof Penderecki, Henri Pousseur, Rodion Shchedrin, Dmitri Shostakovich, Sergei Slonimsky, Karlheinz Stockhausen, Igor Stravinsky, Boris Tishchenko, Anton Webern, and Bernd Alois Zimmermann.