The blood type known as the universal donor is O negative (O-). Individuals with this blood type can donate blood to any other blood type without risking an adverse reaction, making them invaluable in emergency situations where the recipient's blood type may not be immediately known. This is because O negative blood lacks A, B, and Rh antigens, which means it does not trigger an immune response in recipients with different blood types. However, O negative individuals can only receive blood from other O negative donors, highlighting the balance in blood type compatibility.