Blood type refers to the classification of blood based on the presence or absence of certain proteins on the surface of red blood cells. Some synonyms for blood type include blood group, blood classification, and blood phenotype. The ABO system is the most widely used classification, and it recognizes four blood types: A, B, AB, and O. Other systems include the Rh system, which recognizes positive (+) and negative (-) blood types based on the presence or absence of the Rh protein. Identifying a person's blood type is important in blood transfusions and organ transplants, as well as in determining genetic inheritance patterns.