Synonyms for RN (same or very similar meaning)
WordNet sense 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):
radonWordNet sense 2 (a graduate nurse who has passed examinations for registration):
registered nurseFrom the ODE community, based on WordNetadd/edit