Priapism is defined as a prolonged and persistent penile erection lasting >4 hours, unassociated with sexual interest or stimulation.  It is a true medical emergency with complications potentially resulting in permanent erectile dysfunction.  In stuttering or recurrent priapism, individual priapic episodes may last <4 hours.
A classification system has been developed to assist with the practical understanding of priapism and facilitate its clinical management. Priapism has been divided into 3 main categories: ischemic, nonischemic, and stuttering priapism.  Ischemic priapism, also termed veno-occlusive or low-flow priapism, is a persistent erection marked by rigidity of the corpora cavernosa and little or no cavernous arterial inflow. The lack of arterial inflow creates an ischemic environment in the penis whereby abnormalities in the cavernous blood gases are seen. Ischemic priapism is painful and considered a medical emergency.   Nonischemic priapism, also termed arterial or high-flow priapism, is a persistent erection caused by unregulated cavernous arterial inflow.  The penis is tumescent but not fully rigid. The continual arterial inflow does not predispose to an ischemic environment; therefore, cavernous blood gases are normal and typically the erection is not painful.   Nonischemic priapism is not considered a medical emergency but should be treated in a timely manner. The third type, stuttering or recurrent priapism, is characterized by recurring episodes of ischemic priapism. These episodes generally last <4 hours before remission, but they may increase in frequency and/or duration, potentially developing into full ischemic priapism episodes.   Episodes are painful and commonly arise during nocturnal sleep, preceding or following sexual stimulation, and after morning erections. All stuttering episodes that progress to >4 hours in duration should be treated the same as ischemic episodes and considered a medical emergency.  Episodes <4 hours should be treated with the goal of preventing future recurrences in order to decrease ischemic damage to the penis. Ischemic and stuttering priapism resolve when the penis returns to a flaccid and nonpainful state, although in some cases penile edema and partial erection may remain.  Resolution of nonischemic priapism constitutes return to a flaccid state. 
Veno-occlusive or low-flow priapism.
Typically features little or absent intracorporal blood flow.
Cavernous blood gases are abnormal.
May be associated with hematologic abnormalities (e.g., sickle cell disease), malignancy, medication or illicit drug use (psychiatric medications, cocaine, amphetamines), and vasoactive drug use.
Is a true compartment syndrome involving the penis, in which there are characteristic changes in cavernous blood gases and excessive increases in intracorporal pressure.
Penis is fully rigid.
Penile pain is present.
Is a medical emergency and must be treated.
Arterial or high-flow priapism.
Typically features elevated vascular flow within the corpora cavernosa.
Not associated with systemic disease or pharmacotherapy.
Commonly follows an episode of trauma to the perineum or the genitalia.
Penis shows a chronic tolerated tumescence without full rigidity.
Penile pain is not typically present.
Is not a medical emergency.
Recurrent (stuttering) priapism
Usually ischemic/low-flow in nature.
Typically features recurrent attacks of short-lived (<4 hours) priapic episodes.
Often occurs during nocturnal sleep, after morning erections, or preceding or following sexual stimulation.
May be associated with hematologic abnormalities (sickle cell disease).
Is considered a medical emergency and must be treated with a goal of prevention in mind.