If someone has a fit they suddenly lose consciousness and their body makes uncontrollable movements. About two in every five epileptic fits occur during sleep = seizure
If you have a fit of coughing or laughter, you suddenly start coughing or laughing in an uncontrollable way. Halfway down the cigarette she had a fit of coughing
If you say that someone will have a fit when they hear about something, you mean that they will be very angry or shocked. He'd have a fit if he knew what we were up to! = go mad
If you do something in a fit of anger or panic, you are very angry or afraid when you do it. Pattie shot Tom in a fit of jealous rage
Something that happens in fits and starts or by fits and starts keeps happening and then stopping again. My slimming attempts tend to go in fits and starts Military technology advances by fits and starts