Priapism is a prolonged and typically painful erection. That's not associated with sexual stimulation. What makes it dangerous is that eventually the blood that's trapped inside the erect penis doesn't have enough oxygen to keep the tissues healthy. And if it's not urgently addressed permanent damage to the tissues in the penis can result. The word Priapism is named after the Greek God of fertility named Priapus. Now there's no strict cutoff, but typically four hours is cited as the definition of what counts as prolonged.