Selenium is a chemical element. Its symbol is Se. In large amounts it can poison you; living bodies also need a very small amount of it. It was used in older electrical instruments because it reacts to light: when light hits it, the way electricity moves through it changes, so people built parts that could “feel” brightness. You will meet the word in science class more than in daily talk. A Swedish chemist, Jöns Jacob Berzelius, named it in 1818, from the Greek word for the moon. He chose that because this substance had been mixed up with another element, tellurium, whose name comes from the word for Earth — so this one became the “moon” neighbour. It is not silicon (that is a different element, used in computer chips), and it is not “saline” (that means salty). Those only brush it in sound.