VCA
"That's it! Voltage control!"
— Korg MS-20 owner's manual
A VCA, short for voltage controlled amplifier, is exactly what it sounds like — an amplifier controlled by a control voltage. The higher the voltage, the higher the volume.
This control voltage could come from pretty much anything, such as a gate, envelope generator, LFO, or a patch point that lets you use anything to control it.
The signal being amplified needn't be audible — it could itself be another control voltage, so that one control voltage controls the volume of another control voltage. For example, using two VCAs, you can change the volume of the sound over time with each note, and also change the volume of that volume signal based on each note's velocity.
See also
Synthesis: Envelope generator | Footing | Frequency shifter | Noise | Oscillator | Periodic waveform | Program (synthesiser) | Pulsewidth modulation | Sub-oscillator | VCA | VCF | VCO
