A nonmetallic element that resembles sulfur. Metabolically, it is believed to work closely with vitamin E and is an important part of the enzyme glutathione peroxidase. Large amounts of selenium are toxic, however, and selenium poisoning (characterized by emaciation, hair loss, anemia, and bone and liver damage) is often seen in livestock that eat plants which grow in high-selenium soil.
This is based on work I did for the BioTech Dictionary at http://biotech.icmb.utexas.edu/.