Source : Webster's Revised Unabridged Dictionary (1913)

Magnesium \Mag*ne"si*um\, n. [NL. & F. See {Magnesia}.] (Chem.)
   A light silver-white metallic element, 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, 24.4. Specific gravity, 1.75.

   {Magnesium sulphate}. (Chem.) Same as {Epsom salts}.

Source : WordNet®

magnesium
     n : 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: {Mg},
          {atomic number 12}]