Pronunciation: (mag-nē'zē-um, -zhum, -shē-um),[key] —n.Chem. a light, ductile, silver-white, metallic element that burns with a dazzling white light, used in lightweight alloys, flares, fireworks, in the manufacture of flashbulbs, optical mirrors, and precision instruments, and as a zinc substitute in batteries. Symbol: Mg; at. wt.: 24.312; at. no.: 12; sp. gr.: 1.74 at 20°C.