Definition of "selenium" [se•le•ni•um]

• A nonmetallic element, red in powder form, black in vitreous form, and metallic gray in crystalline form, resembling sulfur and obtained primarily as a byproduct of electrolytic copper refining. It is widely used in rectifiers, as a semiconductor, and in xerography. Its photovoltaic and photoconductive actions make it useful in photocells, photographic exposure meters, and solar cells. Atomic number 34; atomic weight 78.96; melting point (of gray selenium) 217°C; boiling point (gray) 684.9°C; specific gravity (gray) 4.79; (vitreous) 4.28; valence 2, 4, or 6. See Table at element. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "As I pointed out (but which whizzed 40,000 feet over your head), selenium is also "necessary for life", but nonetheless a toxic pollutant at higher concentrations (and regulated by the EPA)."
• "If he was just a little bit scientifically inclined, he'd also know that while selenium is an essential cofactor for enzymes necessary to life, it is nonetheless toxic pollutant at high concentrations, and subject to air pollution regulations."
• "I shall probably have to use the metal called selenium, which is very sensitive to light, and which makes a good or a poor electrical conductor according as more or less light falls on it."