What is an aircraft's primary propulsion system called?

The primary propulsion system of an aircraft is referred to as the powerplant. This term encompasses the complete assembly that generates thrust to propel the aircraft, which typically includes the aircraft engine but may also involve additional components such as the propeller or rotor system, fuel delivery system, and other integrated elements necessary for operation.

Using the term "powerplant" is particularly relevant in the context of aviation, where it denotes the entire mechanism that enables flight, rather than simply the engine itself. This term helps differentiate between the various parts involved in producing thrust and those related to other functions of the aircraft.

While "engine" is a component of the powerplant, it does not fully capture the complexity and integration of the systems involved in propulsion. Terms like "thruster" and "compressor" are more specific to certain types of propulsion systems, such as those used in specialized aircraft, but do not broadly apply to all aircraft propulsion systems. Thus, "powerplant" is the most accurate and encompassing term for an aircraft's primary propulsion system.