One of the continuous controller commands available in MIDI. It is one of the original definitions in the MIDI specification that allows for the modulation of synthesizer sounds over time. It is often used to define the action of things like foot pedals, modulation wheels, and sliders on keyboards. As defined by the MIDI specification this controller (number 11) has a range of values from 0 (all the way off) to 127 (all the way on).
Most of the time expression is defined as a subset of Volume (Continuous Controller 7), especially as it relates to natural crescendos and decrescendos by sustained-tone instruments, such as strings, wind, or synth pads. This allows you to set an overall track level using volume and then adjust single notes or groups of notes by increasing or decreasing the expression level. This can be achieved live by using a knob or slider on your synth. In MIDI sequencing there are many ways to insert expression messages into a track.
Sophisticated synths and samplers often incorporate many more elements than volume into expression parameters, to offer maximum sonic control. These can include LFO modulation, increased/decreased sample crossfades (such as the “breathiness” in flute samples) and filter values and resonance.
A little-known MIDI fact is that there are TWO controllers reserved for expression: #11 (coarse) and #43 (fine). In the standard MIDI environment, controller 11 offers 128 divisions of volume or any other parameters assigned to expression. Employing the “fine” adjustment would increase this resolution to 16,384 available steps! Virtually no instrument employs this, although more powerful computers and increased sample resolutions and rates might make this level of control practical.