Someone's blood group is the type of blood that they have in their body. There are four main types: A, B, AB, and O. = blood type. Any of several immunologically distinct, genetically determined classes of human blood that are based on the presence or absence of certain antigens and are clinically identified by characteristic agglutination reactions. Also called blood type. one of the classes into which human blood can be separated, including A, B, AB and O American Equivalent: blood type