Selenium was discovered jointly in 1817 by Swedish chemists Jöns Jakob Berzelius (1779–1848) and Johan Gottlieb Gahn (1745–1818), as an impurity in sulphuric acid. Tellurium had already been discovered, and named after the Greek word for earth, so Berzelius named selenium using the Ancient Greek word σελήνη (selene), or moon. 

Selenium has the atomic number of 34 and an atomic mass of 78.96 AMU (atomic mass units). This element is unique in the fact that it has many different forms. As discovered by Berzelius, selenium can be a red powder, often called an amorphous form, meaning without a crystalline shape.

• Selenium: Selenium exists in a number of allotropic forms. The amorphous forms of selenium do not have specific melting points. ...
• Fun fact about Selenium: Selenium gets its name from the Greek word “selene”, which means “moon”. Selene was the Greek goddess of the moon.
• Chemical symbol: Se.
• Although it is uncommonly found, selenium does exist in relatively pure form, free in nature.
• Selenium is a nonmetal. Like many nonmetals, it exhibits different colors and structures (allotropes) depending on the conditions.
• Brazil nuts are high in selenium, even if they are grown in soil that is not rich in the element. A single nut provides enough selenium to meet the daily requirement for a human adult.
• English electrical engineer Willoughby Smith (1828–1891) discovered that selenium reacts to light (photoelectric effect), leading to its use as a light sensor in the 1870s. Scottish-born American inventor Alexander Graham Bell (1847–1922) made a selenium-based photophone in 1879.
• The primary use of selenium is to decolorize glass, to color glass red, and to make the pigment China Red. Other uses are in photocells, in laser printers and photocopiers, in steels, and in semiconductors.
• There are six natural isotopes of selenium. One is radioactive, while the other five are stable. However, the half-life of the unstable isotope is so long that it is essentially stable. Another 23 unstable isotopes have been produced.
• Some plants require high levels of selenium to survive, so the presence of those plants means the soil is rich in the element.
• Liquid selenium exhibits extremely high surface tension.
• Selenium is important to several enzymes, including antioxidant enzymes glutathione peroxidase and thioredoxin reductase and the deiodinase enzymes that convert thyroid hormones into other forms.
• Approximately 2,000 tons of selenium are extracted annually worldwide.
• Selenium is most commonly produced as a byproduct of copper refining.
• The element was featured in the films "Ghostbusters" and "Evolution."

Just the facts:

Selenium Definition & Meaning - Merriam-Webster