A procedure that temporarily circulates and oxygenates the blood by diverting the blood from the heart and lungs through a machine (called heart-lung machine) and then re-infusing the oxygenated blood into the patient's body (via the aorta) so that surgeries requiring the opening of the heart chambers can be performed.
The cardiopulmonary bypass, as the name implies, is a technique applied in certain heart surgeries that temporarily acts as the "heart" and "lungs" by maintaining the circulation and oxygenation of blood. It does so by connecting the pump to the patient's body.
Word origin: cardio- (heart) + pulmonary (lungs) + bypass
See also: heart-lung machine