The Collaborative International Dictionary of English v.0.48:

Magnesium \Mag*ne"si*um\, n. [NL. & F. See Magnesia.] (Chem.)
   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 sulphate. (Chem.) Same as Epsom salts.
      [1913 Webster]

WordNet (r) 3.0 (2006):

magnesium
    n 1: 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) [syn: magnesium,
         Mg, atomic number 12]