selenium

American  

[si-lee-nee-uhm] / sɪˈli ni əm /

noun

Chemistry.

1. 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. Se; 78.96; 34; (gray) 4.80 at 25°C, (red) 4.50 at 25°C.

---

selenium British  

/ sɪˈliːnɪəm /

noun

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)

---

selenium Scientific  

/ sĭ-lē′nē-əm /

1. 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.

2. See Periodic Table

---

Etymology

Origin of selenium

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

Example Sentences

Bamboo also provides essential amino acids, along with minerals such as selenium and potassium.

From Science Daily • Jan. 16, 2026

He realised the participants receiving benefits struggled to access proper nutrition, including consuming vitamins like vitamin A, iron, zinc, magnesium and selenium.

From BBC • Aug. 7, 2025

Heal the Bay found concentrations of aluminum, iron, selenium and manganese that were higher than drinking water standards, sometimes by 10 times as much.

From Los Angeles Times • Apr. 12, 2025

Others have found that using dandruff shampoo, especially Selsun Blue with selenium sulfide, can be helpful.

From Seattle Times • May 15, 2024

“Why not send him for the selenium direct?”

From "I, Robot" by Isaac Asimov