Rh stands for the Rhesus factor, a protein that can be present on the surface of red blood cells. It is an important component in blood typing, determining whether a person's blood type is Rh-positive or Rh-negative. If a person has the Rh protein, they are considered Rh-positive; if they do not, they are Rh-negative. The presence or absence of the Rh factor can affect blood transfusions and pregnancy. For example, if an Rh-negative mother carries an Rh-positive baby, it can lead to complications if the baby's blood enters the mother's bloodstream. This condition is known as Rh incompatibility.