VCA (voltage-controlled amplifier)
An amplifier whose gain is set by a voltage rather than a knob.
In an analogue synth the VCA sits last in the chain, after the oscillator and the filter. It is what makes a note start and stop: an ADSR envelope sends a voltage that opens and closes the amplifier. Without one the oscillator drones forever, as owners of broken synths can confirm.
The same part found a second career in the studio. A VCA-style compressor, such as the SSL bus compressor, lets a detector ride the gain quickly and predictably. On large consoles, VCA groups control many channels from a single fader without any audio passing through it — the fader only sends a voltage. David Blackmer of dbx designed the VCA circuit in the 1970s that many of those designs built on.