Mastering
The last stop between a finished mix and a release: level, tone and running order are adjusted so the songs sit together and survive whatever format they are headed for.
The name dates from when an engineer cut the original — the master — into a lacquer disc from which vinyl pressings were made. The job was as technical as it was aesthetic: too much low end and the stylus jumped the groove. The CD turned the master into a file with PQ codes marking the track boundaries, and the constraints moved from physics to 16 bits and 0 dBFS.
The toolkit is small and subtle: EQ, compression (often multiband), a limiter and finally dither down to the release bit depth. During the loudness war mastering was mostly about being the loudest record in the changer. Since streaming services started normalising to a LUFS target nobody wins that game any more, and dynamics have slowly been allowed back in.