If you see someone have a convulsion, it can either be scary or funny, depending on whether it's caused by a medical condition or a really funny joke. A convulsion is a jerking, uncontrolled movement.
When a person has a convulsion, it appears as a sudden, violent movement of the body. Convulsions most often happen as a result of a medical condition or illness like epilepsy, but you can also describe a jerky, irregular movement as a convulsion, even if it's caused by hiccups or laughter. A political upheaval or social turmoil can also be called a convulsion, if it's abrupt and violent. The Latin root word is convulsionem, which means "to tear loose."