A remix is an alternative version of a recorded song, made from an original version. This term is also used for any alterations of media other than song (film, literature, beverages etc.).
A remixer uses audio mixing to compose an alternate master recording of a song, adding or subtracting elements, or simply changing the equalization, dynamics, pitch, tempo, playing time, or almost any other aspect of the various musical components. Some remixes involve substantial changes to the arrangement of a recorded work, but many are harmonic, such as creating a "vocal up" version of an album cut that emphasizes the lead singer's voice. Contemporary remixes are often produced in Digital Audio Workstations (DAWs), which allow producers to reorganize tracks and apply various effects.
Songs are remixed for a variety of reasons:
to give a song a second chance at radio and club 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 recording has been lost or degraded
to alter a song to suit a specific music genre or radio format
to alter a song for artistic purposes
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