Though used (too) generically in our industry, this word can mean many subtly different things. The most common (and correct) understanding is that mastering is the process where recorded material is taken from a “master tape” and prepared for duplication in the format of the final release media. Historically this originated with transferring material from tapes to an acetate master disk, which was the first process in making phonograph records. The entire process was as much an art as a science, and as production quality and technology advanced, many practices were developed that helped make better and better sounding records. This advancement included many potential processes of the audio signal such as equalization, compression, limiting, normalization, widening the stereo image, editing fades, and just putting the songs in the correct order. They started out as simple tweaks that had to be done to make audio play correctly on vinyl records, but producers learned that a good mastering engineer could be the difference in how an album ended up sounding as a whole. A well mastered record was better and more consistent in terms of levels and tone quality, which became more and more important as radio airplay and home hi-fi systems became more prevalent in our society.
Nowadays, with the convenience, quality, and affordability of digital audio equipment, many of these steps are done in the recording studio (home or otherwise) where the material was recorded. A significant percentage of the equipment sold at Sweetwater Sound is for this purpose and is being purchased by beginners and pros alike. Still, however, the last few steps of the mastering process, which is very different for CD’s than it was for LP’s (see WFTD archive LP), are done after the material leaves the studio. Sometimes an actual mastering house is used, and other times the “mastered” material is sent directly to a duplication facility where they can also do the final few steps. The lines between how much of it are done in the studio, versus a mastering house, versus the duplication house are very blurry at this point.
More in depth info on mastering can be found at the following Web sites: