A thunderstorm (sometimes referred to as a "convective storm" is defined as SEVERE if it produces

The definition was made to highlight thunderstorms liable to pose an enhanced hazard to life and property.

Two other phenomena associated with thunderstorms also cause a hazard to life and property and are NOT included in this definition.

What are they and why do you think they are not included?

The government agency responsible for apprising the public about the RISK and OCCURRENCE of severe (convective storms) thunderstorms is the National Weather Service's Storm Prediction Center.