Rh type refers to a person's blood group antigen, also known as the Rhesus factor. There are several synonyms for 'Rh type' that are used interchangeably, including Rh factor, Rh blood group system, and Rhesus blood group. The Rhesus system is named after the Rhesus monkey, which was used in early experiments to identify this blood group antigen. A person can either be Rh positive (having the antigen) or Rh negative (lacking the antigen) depending on their genetics. Knowing a person's Rh type is essential when it comes to blood transfusions or pregnancy-related issues, where Rh incompatibility can cause serious complications.