Illustrated Glossary of Organic Chemistry

R: (1) Designation of the absolute configuration of a stereocenter at which the Cahn-Ingold-Prelog priorities of the attached groups decrease in a clockwise manner when viewed along the bond between the stereocenter and the lowest priority group, with the lowest priority group pointing away from the viewer. From Latin rectus, to the right.

Cahn-Ingold-Prelog priorities: HO > CH3CH2 > CH3 > H, so this is (R)-2-butanol.

(2) The gas constant. In SI units, R = 8.3144621 J  K−1mol−1.