selenium

[si-lee-nee-uhm]
noun Chemistry.
a 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; atomic weight: 78.96; atomic number: 34; specific gravity: (gray) 4.80 at 25°C, (red) 4.50 at 25°C.

Origin:
< Neo-Latin (1818) < Greek selḗn(ē) moon + Neo-Latin -ium -ium; named in allusion to its similarity to tellurium

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World English Dictionary
selenium (sɪˈliːnɪəm)
 
n
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)
 
[C19: from New Latin, from Greek selēnē moon; named by analogy to tellurium (from Latin tellus earth)]

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Medical Dictionary

selenium se·le·ni·um (sĭ-lē'nē-əm)
n.
Symbol Se
A nonmetallic element, with red, black, and gray allotropic forms, resembling sulfur and obtained primarily as a byproduct of electrolytic copper refining. Atomic number 34; atomic weight 78.96; melting point (of gray selenium) 221°C; boiling point (gray) 684.9°C; specific gravity (gray) 4.79; (black) 4.28; valence 2, 4, or 6.

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American Heritage
Science Dictionary
selenium   (sĭ-lē'nē-əm)  Pronunciation Key 
Symbol Se
A nonmetallic element that occurs in a gray crystalline form, as a red powder, or as a black glassy material. It is highly photosensitive and can be used to convert light into electricity. Its ability to conduct electricity also increases with higher exposure to light. For these reasons selenium is used in photocopying technology, photography, and solar cells. Atomic number 34; atomic weight 78.96; melting point 217°C; boiling point 684.9°C; specific gravity (gray) 4.79; (red) 4.5; (black) 4.28; valence 2, 4, or 6. See Periodic Table.
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Example sentences
The list includes several vitamins and inorganic minerals as common as salt and
  as rare as selenium or iodine.
Their cores contain paired clusters of atoms such as cadmium and selenium that
  combine to create a semiconductor.
She tosses in multivitamins and the mineral selenium, plus a pair of
  preparations to lower her cholesterol.
Selenium is a trace mineral essential to good health.
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