Cysteine: An amino acid, one of the 20 building blocks of protein. Cysteine can be synthesized by the body and is not essential to the diet. Its key chemical feature is a thiol group that contains sulfur. This thiol group can combine with the thiol group of another cysteine to form a disulfide bridge, which helps structural proteins and enzymes maintain their configuration. Two cysteine molecules linked by a disulfide bridge make up the amino acid cystine. The symbol for cysteine is Cys.