The Rh in Rh factor stands for Rhesus, as in the monkey that was once widely used in medical research.
When Dr. Karl Landsteiner discovered some of the esoteric properties of blood in 1940, he decided to honor the rhesus monkeys that, in the process of making this discovery, were deprived of their freedom, blood, and lives.
The terms Rh factor refers to the D antigen only, which is part of the Rh blood group system, and is the most important blood group system besides the ABO blood group.
Confused yet? We are.