A petit mal is a kind of mild type of epilepsy, or a brief seizure that characterizes this disorder. During a petit mal, the affected person loses and regains consciousness very quickly.
You can call the illness or the seizures petit mal, and it's also used as an adjective to describe the mildness of the seizures. When someone has a petit mal seizure, people around them may not even notice — the only symptom might be a blank facial expression. This is very different from grand mal seizures, which can involve the violent contracting of muscles in the body. Petit mal means "little illness" or "little evil" in French.