Mastering is the term most commonly used to refer to the process of taking an audio mix and preparing it for distribution. There are several considerations in this process: unifying the sound of a record, maintaining consistency across an album, and preparing for distribution.
Consideration also has to be made for how the individual tracks work together when played one after another in an album sequence. Is there a consistent sound? Are the levels matched? Does the collection have a common “character” and play back evenly so that the listener doesn’t have to adjust the volume?
By following recommended best practices in the audio mastering process, it is actually possible to prevent certain artifacts from occurring when the audio is later compressed to a format like MP3 or AAC.