| Abbreviation | Definition |
|---|---|
| RI | Radical Initiator |
| RIA | Rare Isotope Accelerator |
| RIO | Red Iron Oxide |
| RIS | Rotational Isomeric State |
| RISE | Revitalize Iowa's Sound Economy |
| RITG | Radiatively Important Trace Gass |
| RL | Reaction Level |
| RL | Recirculated Liquid |
| RMM | Relative Molecular Mass |
| RMN | (r)-Mandelic Acid |
| RMSEP | Root Mean Square Error of Prediction |
| Rn | Radon |
| RN | Residue Number |
| RNA | RiboNucleic Acid |
| RNS | Reactive Nitrogen Species |
| RNT | Residual Nitrogen Time |
| RO | Red Oxide |
| ROAC | Role of Oceans in Atmospheric Chemistry |
| ROCS | Rapid Overlay Of Chemical Structures |
| rOEF | Relative Oxygen Extraction Fraction |