Sometimes abbreviated CV, Control Voltage is a DC electrical signal used to manipulate the values of components in analog circuits. Control voltages are used in numerous ways in many different types of electronic circuits for all sorts of purposes. A few examples germane to music technology:
If you send a specific electrical voltage to a module of an analog synthesizer (such as an ADSR envelope generator), you can specify what you want the module to do (perhaps lengthen the decay time).
In a mixer and other audio circuit that uses a voltage-controlled amplifier (VCA), a DC voltage can be used to set the gain of the VCA. This is applicable for things like compressors and gates, where the DC voltage may either be a signal proportional to the audio level the devices are acting upon, or could be from some other source. But it also applies to VCA-style mixing automation systems, where a control voltage is set by the user (via some interface, but usually faders) to determine and subsequently log the desired levels in the mixer at specific points in time.
In modern keyboards and synthesizers a control voltage pedal (or other controlling device) can be used to manipulate certain parameters, which, in the case of a pedal, would leave the player’s hands free to play the keyboard. In these cases the pedal usually doesn’t generate the voltage. Instead the pedal is attached to a potentiometer, which acts as a variable resistance to a circuit providing the voltage inside the keyboard. The circuit is able to “read” the position of the pedal by how it acts on the circuit and uses that information to provide data to the specified parameter.