Definition
The combined system of an aircraft's engine (powerplant) together with the propeller or rotor it drives, which converts engine power into the thrust or lift force that moves the aircraft through the air. In fixed-wing aircraft this is the engine and propeller; in helicopters it is the engine and rotor system.
Plain English
The engine plus the spinning blades it turns. The engine makes the power, and the blades turn that power into the push (or lift, in a helicopter) that moves the aircraft.
Context Anchor
Seen in discussions of thrust, especially when explaining how an aircraft produces the force needed to move, climb, or maintain flight.
Derivation
Powerplant comes from the idea of a 'plant' or installation that generates power, the same way a factory's power plant generates electricity. Propeller comes from the Latin propellere, meaning 'to drive forward.' Rotor is short for rotator, something that rotates. Together the term names both the source of power and the device that turns that power into motion through the air.
Why Pilots Care
Correct understanding ensures proper airspeed and climb management during instrument flight.
Intuition Check
Do not read powerplant as a building or factory. In aircraft use, it means the engine system that provides power. Do not assume propeller and rotor are the same part. A propeller is typical on many airplanes; a rotor is typical on helicopters and some rotorcraft.
Example Sentence 1
The powerplant/propeller or rotor combination is what produces the thrust needed to overcome drag in level flight.
Example Sentence 2
In the helicopter, the powerplant drives the rotor to produce both lift and forward thrust.