| Re | Reynold number |
|---|---|
| RHN | Rockwell hardness number |
| RN | radionuclide; red nucleus; Registered Nurse; registry number; residual nitrogen; reticular nucleus |
| Sh | sheep; Sherwood number; Shigella; shoulder |
| s | Greek lower case letter sigma; conductivity; cross section; millisecond; molecular type or bond; pop... |