Short for magnetic film, a mag is where audio is temporarily stored during the post production process. A mag looks exactly like motion picture film, but with no picture on the base stock, just the usual oxide (see WFTD archive Oxide) that you find on any recording tape (the uninitiated might even look at it and figure it to be raw film stock for filming the picture). In film production the audio is transferred from analog or DAT (whatever mediums were used) to mag for the post production and editing process.
