A remix is a piece of media which has been altered or contorted from its original state by adding, removing, and changing pieces of the item. A song, piece of artwork, books, video, poem, or photograph can all be remixes. The only characteristic of a remix is that it appropriates and changes other materials to create something new.
Most commonly, remixes are a subset of audio mixing in music and song recordings. Songs may be remixed for a variety of reasons:
to adapt or revise a song for radio or nightclub play
to create a stereo or surround sound version of a song where none was previously available
to improve the fidelity of an older song for which the original master has been lost or degraded
to alter a song to suit a specific music genre or radio format
to use some of the same materials, allowing the song to reach a different audience
to alter a song for artistic purposes