Usually abbreviated to ”mod wheel”.
A device found on most synthesizer and MIDI controller keyboards, usually to the left of the keys next to the pitch bend wheel. When playing a synthesizer that accepts mod wheel controller signals, turning the wheel changes (modulates) the sound in some way. On older synthesizers this modulation usually consisted of adding tremolo to the sound by applying a LFO to modulate the sound, hence the name.
On more modern instruments the modulation can change one or several parameters including filters, cut, resonance, effect send levels, ADSR-parameters, and just about anything else that makes the sound interesting. Using combinations of these parameters linked to the mod wheel helps the musician to do complex effects without having to twist several knobs at once. This is especially helpful when playing live.
On a standard MIDI keyboard, the mod wheel sends controller message 01, with values ranging from 0 to 127.