a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds. Selenium (Greek se???? selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously-known tellurium (named for the Earth).

Selenium occurs in (fairly) pure form in nature. This native selenium is exactly as it came out of the ground. It is used in many light-sensing applications: photocopiers, automatic light switches and light meters.