A propeller is a mechanical device that converts rotational motion into thrust, allowing vehicles like airplanes and boats to move through air or water. It typically consists of blades that spin around a central hub, creating a difference in pressure that pushes the vehicle forward. Propellers are commonly made from materials such as metal or composite materials for durability and efficiency. They are essential components in various modes of transportation, including aircraft, ships, and submarines, and their design can vary significantly based on the specific requirements of each application.