Blood group: An inherited feature on the surface of the red blood cells. A series of related blood types constitutes a blood group system, such as the Rh or ABO system. The frequencies of the ABO and Rh blood types vary from population to population. In the US, the most common type is O+ (meaning O in the ABO system and positive in the Rh system), which is present in 37.4 percent of the population. The frequencies in the US (in descending order) are O+ (37.4 percent), A+ (35.7 percent), B+ (8.5 percent), O- (6.6 percent), A- (6.3 percent), AB+ (3.4 percent), B- (1.5 percent), and AB- (0.6 percent).