Illustrated Glossary of Organic Chemistry

Sulfoxide: A functional group characterized by a sulfur atom bearing one lone pair, one double bond to oxygen, and two single bonds to carbon. When the carbon groups are different the sulfur atom is a stereocenter.

Dimethylsulfoxide (DMSO), a polar aprotic solvent. An older notation for the highly polar S-O pi bond is an arrow, starting at the less electronegative element (sulfur EN = 2.5) and pointing toward the more electronegative element (oxygen EN = 3.5).