Priapism is a prolonged and often painful erection that lasts more than a few hours, even days. It is not associated with arousal or desire. Blood flows into the penis, but is unable to drain off. The condition is named after Priapus, the Greek god of fertility.
There are several possible causes. It can for instance be caused by aphrodisiacs or drugs (cocaine, marijuana, ecstasy, antidepressants), be an allergic reaction to injections to treat erection problems, result from penile trauma or be caused by sickle cell disease, leukaemia, malaria, spiral cord injury or by a black widow spider bite.
If blood gets trapped in the penis for too long damage may occur to the erectile compartment, resulting in a loss of the ability to achieve erections. So if this happens to you consider it an emergency and seek help (within 4-6 hours!).
Priapism should not be confused with spontaneous erections.