The Collaborative International Dictionary of English v.0.48:

Strontium \Stron"ti*um\, n. [NL. See Strontia.] (Chem.)
   A metallic element of the calcium group, always naturally
   occurring combined, as in the minerals strontianite,
   celestite, etc. It is isolated as a yellowish metal, somewhat
   malleable but harder than calcium. It is chiefly employed (as
   in the nitrate) to color pyrotechnic flames red. Symbol Sr.
   Atomic weight 87.3.
   [1913 Webster]

WordNet (r) 3.0 (2006):

strontium
    n 1: a soft silver-white or yellowish metallic element of the
         alkali metal group; turns yellow in air; occurs in
         celestite and strontianite [syn: strontium, Sr, atomic
         number 38]