noun

• (medicine) Abbreviation of Rhesus factor. [An antigen of the Rhesus system; understood unless otherwise specified to refer to Rhesus factor D (RhD), which appears on the surface of the red blood cells of most (85%) humans. An incompatibility between the Rh-blood types of a mother and her baby may cause serious disease for the baby.]
• (medicine) Abbreviation of Rhesus. [(medicine, attributive, also without cap.) Denotes a system for categorising human blood types based on the presence or absence of various antigens, especially Rhesus factor D.]
• Initialism of relative humidity. [(meteorology) The ratio of the actual amount of water vapor (absolute humidity) present in the air to the saturation point at the same temperature, usually expressed as a percentage.]
• (music, knitting) Initialism of right hand.
• Initialism of review hearing.
• (US, army, aviation) Initialism of reconnaissance helicopter.