An adjustable-pitch propeller is a type of propeller used in marine or aviation applications that allows the pitch or angle of the blades to be adjusted. It is typically used in situations where variable speed or direction of travel is required, and can save fuel, reduce noise and vibration, and improve efficiency. The pitch can be adjusted manually or automatically based on engine speed and other factors. Adjustable-pitch propellers are commonly used in aviation for turboprop engines and in marine applications for diesel and gas engines.