Mixing is the stage after the recording. It is the art of enhancing the sound of every element separately and combining all recorded tracks to fit in a sound field. Mixing Equipment includes EQs, compression, panning, reverb, delays, and other effects used for this purpose.
Mastering is the stage after Mixing. It is the art of enhancing the quality of the entire mix, taking it a step further sonically to be ready for release, making sure it sounds well through multiple playback systems and in different formats.
In case of an album or EP, all the tracks will have a consistent perceived loudness, Insertion of ISRC Codes, PQ codes, CD-Text, DDP, Apple Digital Master Formats, etc. More about Mastering
Midfield (moslty passive) Speakers are always used in Mastering with high quality crystal clear AD/DA Converter so as to reflect all frequencies and mimic a typical home listening situation. Monitors should be around 2 meters far from the engineer. Ideally, there should be No Obstruction between the Monitors and Engineer.
Nearfield monitors are best suitable for recording and mixing studios for precise instrument localization and reverb/effects judgment.
Analog high-end processors are used in Mastering for best sonic results ( quality, noise, headroom) . The gears are often custom-made and carefully chosen based on each Mastering engineer's taste (various tube valves and other technical features).
Digital plugins are rarely used in Mastering (only a few limiting is needed) whereas Mixing nowadays relies on digital processing (especially in home studios) with the settings on all tracks are recalled by a single click within the session.
A special DAW ( Digital Audio Worksation ) is used for Mastering with an extended algorithmic calculation including features like insertionn of ISRC, PQ Codes, CD-Text, DDPi delivery, formats...
Mixing DAWs are moslty designed for ease of use in editing, automation, plugins insertion and mixing.
Early reflections and Noise in Mastering are kept at minimum for a precise judgment without any barrier between the monitors and the engineer. An inclined ceiling is a plus to reflect frequencies behind the listening position.
A dedicated room treatment with specific specs (NC, ER, Reverb Time) is a must in Mastering.
Mixing Studios often include a large console or desk which is important and practical (but has its drawbacks...). The mixing room is often surrounded by machines/PC/Sofas and is connected to a recording room with glass etc...
Mastering Engineer gets trained through time to listen in a macro perspective, concentrating on the whole stereo track as one group.
A fresh set of Healthy ears with a creative mind are essential in Mastering to snatch any minor issue since this is the last stage before releasing the Recording!
Mix Engineer gets trained to listen in a micro perspective, concentrating on each track, the balance between each track and group, and the stereo image...
So, Since Mastering is about Enhancing the Quality of the Entire Mix, Wouldn't it be more Logical to Enhance the Quality of this Mix during Mixing?