Plain English Meaning
Thunderstorm activity that is dangerous to fly through or near — strong storms, lines of storms, storms hidden in cloud layers, storms with large hail, or storms producing very strong surface winds.
Full Definition
Thunderstorm-related weather that poses a serious threat to aircraft, including severe or embedded thunderstorms, lines of thunderstorms, thunderstorms producing hail three-quarters of an inch or greater in diameter, and thunderstorms producing surface winds of 50 knots or greater. It is the type of weather that triggers a Convective SIGMET (WST).