1. (rhesus factor) An antigen that may or may not be present on the surface of human red blood cells, first identified in rhesus macaques. A person with blood possessing the antigen is said to be rhesus-positive (Rh+), one lacking it rhesus-negative (Rh–). Most people are Rh+. If Rh+ blood is given to an Rh– person it can lead to blood-clotting and a pregnant Rh– woman carrying an Rh+ foetus may cause anti-Rh antibodies to develop in the foetus leading to anaemia in the baby.

2. (rhesus macaque, Macaca mulatta) A brown or grey monkey (family Cercopithecidae) with a pink, hairless face that occurs in a wide range of habitats, including close to human habitations, in central, southern, and south-eastern Asia. They often form large troops. Rhesus macaques swim well.