A spinning blade propulsion system used in air and water travel. The blades are angled so they push the medium(that is the air or water - both refered to also as fluids in this context) in a direction continuously.

Helicopters use a very specialised propellor in which each individual blade can be rotated and positioned as they rotate. They take their name from the fact that the force in a Propellor or rotor is Helical - that is spiral like.