Hysterical describes two very different things: being so overwhelmed by emotion that you lose control, or being wildly, almost uncontrollably funny.

In its serious meaning, someone becomes hysterical when fear, panic, or grief becomes so intense that they can't think clearly or calm down. During a fire drill, a hysterical student might start screaming and not hear the teacher's instructions to line up calmly. After a frightening accident, someone might become hysterical with worry, sobbing and shaking uncontrollably.

But in everyday conversation, people usually mean the funny version. When your friend tells a joke so hilarious that you can barely breathe from laughing, you might say “That's hysterical!” A comedy show can be hysterical if it makes the whole audience roar with laughter. This meaning captures that same sense of losing control, but in a joyful way: you're laughing so hard you can't stop.

The context usually makes clear which meaning someone intends. “The movie was hysterical” means it was hilarious, while “She became hysterical during the storm” suggests genuine fear or panic. The word hysteria refers specifically to the serious meaning: a state of uncontrollable emotion.