(1) (noun) The composite blend of live sounds and/or recorded virtual tracks to a new program, usually of fewer tracks and often to a different recording/storage medium.
(2) (verb) To produce the new composite program, adding the live sounds, recorded virtual tracks, effects and other processing such as equalization, compression, reverb, etc., ready for the production of a recording of the desired number of tracks and on the desired recording and/or storage medium, possibly for use as a master.