The Collaborative International Dictionary of English v.0.48:

Calcium \Cal"ci*um\ (k[a^]l"s[i^]*[u^]m), n. [NL., from L. calx,
   calcis, lime; cf F. calcium. See Calx.] (Chem.)
   An elementary substance; a metal which combined with oxygen
   forms lime. It is of a pale yellow color, tenacious, and
   malleable. It is a member of the alkaline earth group of
   elements. Atomic weight 40. Symbol Ca.
   [1913 Webster]

   Note: Calcium is widely and abundantly disseminated, as in
         its compounds calcium carbonate or limestone, calcium
         sulphate or gypsum, calcium fluoride or fluor spar,
         calcium phosphate or apatite.
         [1913 Webster]

   Calcium light, an intense light produced by the
      incandescence of a stick or ball of lime in the flame of a
      combination of oxygen and hydrogen gases, or of oxygen and
      coal gas; -- called also Drummond light and lime
      light.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

calcium
    n 1: a white metallic element that burns with a brilliant light;
         the fifth most abundant element in the earth's crust; an
         important component of most plants and animals [syn:
         calcium, Ca, atomic number 20]