WordReference Random House Unabridged Dictionary of American English © 2019
se•le•ni•um  (si lēnē əm),USA pronunciation n. [Chem.]
  1. Chemistry, Nutritiona nonmetallic element chemically resembling sulfur and tellurium, occurring in several allotropic forms, as crystalline and amorphous, and having an electrical resistance that varies under the influence of light. Symbol: Se;
    at. wt.: 78.96;
    at. no.: 34;
    sp. gr.: (gray) 4.80 at 25°C, (red) 4.50 at 25°C.
  • Greek selé̄n(ē) moon + Neo-Latin -ium -ium; named in allusion to its similarity to tellurium
  • Neo-Latin (1818)

Collins Concise English Dictionary © HarperCollins Publishers::

selenium /sɪˈliːnɪəm/ n
  1. a nonmetallic element that exists in several allotropic forms. It occurs free in volcanic areas and in sulphide ores, esp pyrite. The common form is a grey crystalline solid that is photoconductive, photovoltaic, and semiconducting: used in photocells, solar cells, and in xerography. Symbol: Se; atomic no: 34; atomic wt: 78.96; valency: –2, 4, or 6; relative density: 4.79 (grey); melting pt: 221°C (grey); boiling pt: 685°C (grey)
Etymology: 19th Century: from New Latin, from Greek selēnē moon; named by analogy to tellurium (from Latin tellus earth)

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