A propeller is a piece of equipment with two or more blades that spin around a centre. The spinning pushes air or water backwards, and that push moves an aircraft or a ship forwards. Small planes often have a propeller on the nose; boats have one under the water at the back. The word is ordinary technical English — not fancy, not slang. Though actually, a ceiling fan is not a propeller, even though the shape looks similar; we keep propeller for the thing whose job is to move the vehicle. A big jet airliner usually has no propeller you can see; jet engines work another way. People also say “the props” in casual talk about planes, but the full word is the one you want in a textbook.