seaborgium (symbol Sg) Synthetic, radioactive, metallic element of the transactinide series. It has a half-life of less than a second. It was thought to have been discovered in 1974 by US nuclear scientist Albert Ghiorso (1915– ) and colleagues at the University of California at Berkeley, but its existence was only proved in 1994. It was named in 1999 after US nuclear chemist Glenn Seaborg (1912–99), having first been temporarily named unnilhexium. Properties: at.no. 106.

sea·bor·gi·um / sēˈbôrgēəm/ • n. the chemical element of atomic number 106, a very unstable element made by high-energy atomic collisions. (Symbol: Sg)