- selenium, Se, atomic number 34 - a toxic nonmetallic element related to sulfur and tellurium; occurs in several allotropic forms; a stable gray metallike allotrope conducts electricity better in the light than in the dark and is used in photocells; occurs in sulfide ores (as pyrite)
--1 is a kind of chemical element, element; antioxidant
--1 is a substance of pyrite, iron pyrite, fool's gold