A "Universal Donor" refers to a person with blood type O negative. This blood type can be transfused to patients of any other blood type without causing an adverse reaction. This is crucial in emergency situations where there may not be time to determine a patient's specific blood type. Individuals with O negative blood are especially valuable in medical settings, as they can help save lives during surgeries, trauma care, and other critical situations. However, while they can donate to anyone, they can only receive blood from other O negative donors, making their own blood type relatively rare.