A device consisting of a pivoting foot pedal tied to some circuit which generates a control signal, depending on the position of the pedal. Some are self-powered and generate a control voltage output. Others are intended to be plugged into a dedicated input on a synth; generally the synth supplies power and ground via a TRS jack, and the pedal circuitry is just a potentiomenter tied into the mechanism.
A MIDIcontinuous controller type. Synths which have dedicated expression-pedal inputs often output controller messages of this type when the foot pedal position is changed.