Plain English Meaning
A clear, obstacle-free area off the end of the runway that an airplane can climb over during the first part of its takeoff. It is not part of the runway itself, but it is officially recognized as safe to climb across, so it can be counted when figuring out how much room the airplane has to get airborne.
Full Definition
An area beyond the runway, centered on the extended runway centerline, that is under the control of airport authorities and is selected or prepared as a suitable area over which an airplane may make a portion of its initial climb to a specified height. For turbine-powered transport category airplanes certificated after September 30, 1958, the clearway plane extends from the end of the runway with an upward slope not exceeding 1.25 percent, with no protruding , and a minimum width as specified in .