Multitrack audio production is generally broken down into three engineering phases:
TRACKING - A fancy word for recording. The tracking phase is the first phase where all recording is done.
MIXING - This is the middle stage where all the individual instruemt tracks and takes are edited together, sonically refined and summed - or mixed together - into a stereo (or surround sound, if that's your game) version. This version is usually called the "mixdown".
MASTERING - This is where the mixdown gets it's final sonic "polish" and where it is combined with other polished mixdowns into an album of material and made into a CD.
Technically the polishing part is called "premastering", and the assembly of a final master CD is the "mastering". But in home recording cases where there may be no CD compilation, and/or where everything is done on a basic level on a home computer by a single person, the whole process after mixdown is just referred to simply as "mastering".
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