A propeller is a mechanical device that converts rotational motion into thrust, allowing vehicles like boats and airplanes to move through water or air. It typically consists of blades that spin around a central hub, creating a difference in pressure that pushes the vehicle forward. Propellers can be found in various sizes and designs, depending on their application. For example, airplane propellers are designed for high-speed flight, while boat propellers are optimized for efficiency in water. Both types play a crucial role in the performance and maneuverability of their respective vehicles.