Princeton's WordNet

1. rhesus factor, Rh factor, Rh(noun)

   a blood group antigen possessed by Rh-positive people; if an Rh-negative person receives a blood transfusion from an Rh-positive person it can result in hemolysis and anemia

2. rhodium, Rh, atomic number 45(noun)

   a white hard metallic element that is one of the platinum group and is found in platinum ores; used in alloys with platinum

3. releasing hormone, RH, releasing factor, hypothalamic releasing hormone, hypothalamic releasing factor(noun)

   any of several hormones produced in the hypothalamus and carried by a vein to the anterior pituitary gland where they stimulate the release of anterior pituitary hormones; each of these hormones causes the anterior pituitary to secrete a specific hormone