(1) Refers to the artistic touchup of the system’s response, as might be done by the mix engineer to tailor the response of the system to taste; this is done by ear or perhaps aided by low-resolution instrumentation such as a real-time analyzer.
(2) One or more knobs on an audio amplifier or preamp which modify the relative balance between the treble and bass tones. Tone controls are actually equalizers and they change the frequency response curve of the device implementing them.