Priapism is when a man's pee-pee (penis) stays hard for a very long time, even when he doesn't want it to be hard.
It happens because the blood stays in the pee-pee, and doesn't flow back out like it should. This can be very painful and uncomfortable for the man, and can even damage his pee-pee if it lasts too long.
Priapism can be caused by different things, like sickle cell disease, medications, or trauma to the pee-pee. If a man has priapism, it's important for him to see a doctor right away so they can help him get better.