Hysteria is a medical condition or general state of extreme fear and panic. When hysteria sets in, people are out of control.
In a medical sense, people who are feeling hysteria may be violent and having trouble perceiving reality. In a more general sense, when someone is hysterical, they're out of control — freaking out. Hysteria is the opposite of calmness. It's really a state of emotional chaos that often takes over during natural disasters and scary moments. Or if you're a 13 year old girl, when you go to a concert for your favorite boy band.