A convulsion is a sudden, violent shaking of the body that a person cannot control. During a convulsion, muscles tighten and jerk rapidly and forcefully, making the person's arms, legs, or whole body thrash and spasm. Convulsions happen when something disrupts the normal electrical signals in the brain, most commonly during certain types of seizures.

Watching someone have a convulsion can be frightening. Doctors can often control convulsions with medicine. Some people have conditions that make convulsions more likely, while others might have one convulsion in their lifetime and never have another.

The word can also describe any violent shaking or disruption. A country experiencing a convulsion of political upheaval is being torn apart by rapid, uncontrollable change. An engine might shake in convulsions before it finally stops working. When used this way, convulsion suggests chaos and loss of normal order, like the uncontrolled muscle movements the word originally described.