The most polpularly known blood grouping is the ABO grouping. It is named ABO and not ABC, because, because "O" in it refers to having

Landsteiner divided human population into four groups based on the presence of antigens found in their RBCs. Each group represented a blood group . Thus, there are four types of blood group A,B,AB and O. Blood group 'O' does not contain any antigen on RBCs, hence can be given to any person, that's why this blood group is called universal donor.