The term "O-" refers to a specific blood type in the ABO blood group system. It is characterized by the absence of A and B antigens on the surface of red blood cells. Individuals with this blood type can donate blood to any other blood type, making them universal donors, but they can only receive blood from other O- donors. O- blood is particularly important in emergency situations where blood type compatibility is crucial. It is often in high demand due to its universal donor status. Blood banks and hospitals keep a close watch on their O- supplies to ensure they can meet the needs of patients requiring transfusions.