selenium n.

a name of the moon goddess, equivalent to Latin Luna, from Greek selēnē "the moon; name of the moon goddess," related to selas "light, brightness, bright flame, flash of an eye." This is reconstructed to be from PIE root *swel- (2) "to shine, beam" (source also of Sanskrit svargah "heaven," Lithuanian svilti "to singe," Old English swelan "to be burnt up," Middle Low German swelan "to smolder") and to be related to swelter and sultry.

Daughter of Hyperion and Theia, sister of Helios. Related: Selenian "of or pertaining to the moon as a world and its supposed inhabitants," 1660s. Another early word for "moon-man, supposed inhabitant of the moon" is selenite (1640s); Greek had selēnitai "moon-dwellers, the men in the moon" (Lucian).

metallic element, 1800, coined 1798 in Modern Latin by German chemist and mineralogist Martin Heinrich Klaproth (1743-1817) from Latin tellus (genitive telluris) "earth" (see tellurian). With metallic element ending -ium.

word-forming element in chemistry, used to coin element names, from Latin adjectival suffix -ium (neuter of -ius), which formed metal names in Latin (ferrum "iron," aurum "gold," etc.). In late 18c chemists began to pay attention to the naming of their substances with words that indicate their chemical properties. Berzelius in 1811 proposed forming all element names in Modern Latin. As the names of some recently discovered metallic elements already were in Latin form (uranium, chromium, borium, etc.), the pattern of naming metallic elements in -ium or -um was maintained (in cadmium, lithium, plutonium, etc.; helium is an anomaly).