Plain English Meaning

It is the lever the pilot uses to choose how fast the propeller spins. The pilot picks the RPM, and the propeller's internal mechanism keeps it there by changing the angle of the blades.

Full Definition

A cockpit control, found on airplanes equipped with a constant-speed propeller, that sets the desired propeller speed in revolutions per minute (RPM) by adjusting blade pitch through the propeller governor. Moving the control forward selects a higher RPM (flatter blade pitch); moving it aft selects a lower RPM (coarser blade pitch). It is typically the middle lever in a three-lever quadrant, between the throttle and the mixture.