Plain English Meaning
It is the part that physically opens and closes the waste gate. By doing so, it decides how much exhaust gas spins the turbocharger, which in turn controls how much extra power the engine produces.
Full Definition
A device, typically powered by engine oil pressure under the control of the turbocharger pressure controller, that opens and closes the waste gate to regulate how much exhaust gas is routed through the turbocharger turbine. By moving the waste gate, the actuator controls turbine speed and therefore the amount of compressed air the turbocharger delivers to the engine.
Context Anchor
Seen in turbocharged engine system descriptions, especially when discussing control, protection, and turbocharger .