| Acronym | Meaning |
| R-CNDO/1X | Relativistic Complete Neglect Of Differential Overlap (method) |
| R-GVB | Resonating-generalized Valence Bond (wave Function) |
| R2PI | Resonant Two-photon Ionization |
| RA | Reduced Aggregate (interplanetary Dust Particle) |
| RA | Remedial Action |
| RAA | Rigid-atom Approxn. (in Electronic Structure Calcns.) |
| RAAS | Renin-angiotensin-aldosterone System |
| RABITS | Rolling-assisted Biaxially Textured Substrates |
| RAC | Reference Air Concentration (USEPA) |
| RACE | Rapid Amplification Of CDNA Ends |
| RACE | Reactive Accelerated Cluster Erosion |
| RACER | Research On Antarctic Coastal Ecosystem Rates |
| RACI | Royal Australian Chem. Institute |
| RACM | Reasonably Available Control Measures |
| RACM | Regional Atm. Chem. Mechanism |
| RACS | Response Action Contractors |
| RACS | Raw Agricultural Commodities |
| RACT | Reasonably Available Control Technology |
| RADM | Regional Acid Deposition Model |
| RADREM | Radioactivity Research And Environmental Monitoring Committee |
| RADW | Rotationally-adiabatic Distorted-wave |
| RAF | Reactivity Adjustment Factor |
| RAFA | Rank Annihilation Factor Analysis |
| RAFT | Reversible Addition-fragmentation Chain Transfer [polymerization] |
| RAG | Risk Assessment Guideline |
| RAGE | Receptor For Advanced Glycation Endproducts |
| RAGS | Risk Assessment Guidelines For Superfund Sites |
| RAHB | Resonance-assisted Hydrogen Bond |
| RAILE | Retro-Assisted Imaging Laser Experiment |
| RAIN | Reactions And Intermediates Network |
| RAINBOWS | Reduced And INternally Biased Oxide Wafers |
| RAINS | Regional Acidification Information And Simulation |
| RAIRS | Reflection/absorption Infrared Spectroscopy |
| RAL | Recommended Allowable Limit |
| RALLS | Right-angle Laser Light Scattering |
| RAM | Random Access Memory |
| RAM | Reference-system Average Mayer-function (in Perturbation Theory) |
| RAM | Renormalized Atom Model (for Electronic-structure Calcns.) |
| RAM | Restricted Area Molding |
| RAMAN | Regional Atmospheric Measurement And Analytical Network |
| RAMC | Risk Assessment And Management Commission |
| RAMP | Receptor Activity Modifying Protein |
| RAMS | Regional Air Monitoring System |
| RAN | Residual Acrylonitrile |
| RANTES | Regulated On Activation Normal T Cell Expressed And Secreted |
| RAO | Remedial Action Objective |
| RAP | Receptor-associated Protein |
| RAP | Remedial Action Project Or Plan |
| RAPD | Randomly Amplified Polymorphic DNA |
| RAPID | Ruggedized Advanced Pathogen Identification Device |