Plain English Meaning
A thunderstorm cloud. It builds upward into a towering shape, often topped with a flat anvil, and contains the most violent weather a pilot can encounter.
Full Definition
A tall, dense, vertically developed cloud associated with thunderstorms, characterized by strong updrafts and downdrafts, heavy precipitation, lightning, and often hail, severe turbulence, and icing. It typically extends from a low base to very high altitudes, sometimes reaching the tropopause, with an anvil-shaped top when fully mature.