It is popularly referred as "The Rh factor".

 The Rh factor is a protein that can be found on the surface of red blood cells. If your blood cells have this protein, you are Rh positive. If your blood cells do not have this protein, you areRh negative.

It is particularly important as a cause of hemolytic disease of the newborn and of incompatibility in blood transfusions.

The name is derived from the fact that the blood antigen was first described in the Rhesus monkey.