An aircraft that achieves forward propulsion by expelling a stream of fluid is called a jet engine.

Here's why:

* Jet engines work by taking in air, compressing it, burning fuel to heat it, and then expelling the hot gas at high speed. This expulsion of hot gas creates thrust, propelling the aircraft forward.

* The term jet refers to the stream of fluid being expelled, and engine indicates it's the power source for the aircraft.

While other types of engines, like rocket engines, also use the principle of expelling fluid for propulsion, they are not typically considered jet engines. This is because rocket engines carry their own oxidizer (usually liquid oxygen) and are not reliant on atmospheric air like jet engines.