To convulse is to have spasms. Get help immediately if you see someone convulse. This medical condition can be brought on by something simple (like a rise in body temperature) or something more complicated, such as epilepsy.
In addition to being a symptom of a medical condition, to convulse is often applied to people shaking with laughter. So if you go to a club and see a hilarious comedian, you may convulse with laughter so hard that you might cry, or not be able to catch your breath, and your sides hurt.