magnesium(n = noun.substance) atomic number 12, mg - a light silver-white ductile bivalent metallic element; in pure form it burns with brilliant white flame; occurs naturally only in combination (as in magnesite and dolomite and carnallite and spinel and olivine);
is a kind of metal, metallic element

   A light silver-white metallic element of atomic number 12, malleable and ductile, quite permanent in dry air but tarnishing in moist air. It burns, forming (the oxide) magnesia, with the production of a blinding light (the so-called magnesium light) which is used in signaling, in pyrotechny, or in photography where a strong actinic illuminant is required. Its compounds occur abundantly, as in dolomite, talc, meerschaum, etc. Symbol Mg. Atomic weight, 24.305. Specific gravity, 1.75. [1913 Webster]

magnesium, n. Chem. a silvery metallic element occurring naturally in magnesite and dolomite, used for making light alloys and important as an essential element in living organisms.

Idiom:
magnesium flare (or light) a blinding white light produced by burning magnesium wire.

Usage:
Symb.: Mg.

magnesium bicarbonate, magnesium carbonate, magnesium hydroxide, magnesium nitride, magnesium oxide, magnesium sulfate