A sulfoxide is a molecule that has a sulfur atom bonded to two carbons and one oxygen atom. It comes from oxidising thioethers, without going all the way to the sulfone. Like many molecules containing sulfur, sulfoxide can sometimes smell a lot.
Sulfoxides can be written with the general formula R–S(=O)–R'. Sulfur has a double bond with the oxygen, and single bonds with the carbons. That leaves a lone pair on the atom. This means that the shape at that atom is tetrahedral. It also means that the sulfur can be a chiral centre. Sulfoxides are used as good ligands for transition metals.