The blood type considered the universal donor is O negative (O-). This blood type can be given to individuals of any blood type in emergency situations because it lacks A and B antigens on the surface of red blood cells. Consequently, O- blood is essential for blood transfusions, particularly in cases where the recipient's blood type is unknown. Its universal compatibility makes it a vital resource in medical emergencies and surgeries, emphasizing the importance of O- donors in maintaining blood supplies.