Transfusion: The transfer of blood or blood products from one person (the donor) into the bloodstream of another person (the recipient). In most situations, transfusion is done as a lifesaving maneuver to replace blood cells or blood products lost through severe bleeding. Transfusion of one's own blood (autologous transfusion) is the safest method, but it requires advanced planning, and not all patients are eligible. Directed donor blood allows the patient to receive blood from known donors. Volunteer donor blood is usually most readily available and, when properly tested, has a low incidence of adverse events.