wikipedia : A remix is a piece of media which has been altered from its original state by adding, removing and changing pieces of the item. A song, piece of artwork, books, video, 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

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to provide additional versions of a song for use as bonus tracks or for a B-side, for example, in times when a CD single might carry a total of 4 tracks
to create a connection between a smaller artist and a more successful one, as was the case with Fatboy Slim’s remix of “Brimful of Asha” by Cornershop

to improve the first or demo mix of the song, generally to ensure a professional product.
to provide an alternative version of a song
to improve a song from its original state

Remixes should not be confused with edits, which usually involve shortening a final stereo master for marketing or broadcasting purposes. Another distinction should be made between a remix, which recombines audio pieces from a recording to create an altered version of a song, and a cover: a re-recording of someone else’s song like Mike D’s remix of Moby’s “Natural Blues”.

While audio mixing is one of the most popular and recognized forms of remixing, this is not the only media form which is remixed in numerous examples. Literature, film, technology, and social systems can all be argued as a form of remix