The Collaborative International Dictionary of English v.0.48:

Rn \Rn\, n. (Chem.)
   the symbol for the chemical element radon.
   [PJC]

WordNet (r) 3.0 (2006):

Rn
    n 1: a radioactive gaseous element formed by the disintegration
         of radium; the heaviest of the inert gasses; occurs
         naturally (especially in areas over granite) and is
         considered a hazard to health [syn: radon, Rn, atomic
         number 86]
    2: a graduate nurse who has passed examinations for registration
       [syn: registered nurse, RN]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "RN":
   LPN, Naval Construction Battalion, Royal Navy, Seabees, USN,
   United States Navy, argosy, armada, charge nurse, coast guard,
   district nurse, division, escadrille, fleet, flotilla,
   graduate nurse, licensed practical nurse, marine, merchant fleet,
   merchant marine, merchant navy, mosquito fleet, naval forces,
   naval militia, naval reserve, navy, nurse, nursing sister,
   practical nurse, private-duty nurse, probationer, probationist,
   probe, public health nurse, registered nurse, school nurse,
   scrub nurse, sister, squadron, student nurse, surgical nurse,
   task force, task group, trained nurse, visiting nurse