The term MIDI controller can be used literally or point to a particular scenario of how a device is used.

• Used literally, MIDI controller refers to any hardware or software that generates and transmits MIDI data to MIDI-enabled devices.
• in practice, MIDI controller refers to a device that controls parameters of a performance. A slider assigned to open and close a low-pass filter on a synthesizer may be assigned to controller 18, for example. Changes in the position of the slider are transmitted along with "18" so that they are distinguished from changes in the value of other controllers. The MIDI Controller can be populated with any number of sliders, knobs, buttons, and other sensors, and may or may not include a piano-keyboard.