One of a group of compounds that contain both selenium (Se) and carbon (C) and frequently other elements as well, for example, halogen, oxygen (O), sulfur (S), or nitrogen (N). The first examples of organoselenium compounds were reported shortly after the discovery of elemental selenium by J. J. Berzelius in 1818. Their exceptionally unpleasant and persistent odors may have discouraged early investigators. However, less volatile derivatives containing aryl substituents or selenium in higher oxidation states are more easily handled.