Illustrated Glossary of Organic Chemistry

S: (1) Designation of the absolute configuration of a stereocenter at which the Cahn-Ingold-Prelog priorities of the attached groups decrease in a counterclockwise 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 sinister, to the left.

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

(2) Indicates an attachment to a sulfur atom. Compare with N, which means attachment to a nitrogen atom.

S-adenosyl methionine (SAM)
S means the adenosine group is bonded to the sulfur atom of methionine.