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]

The Elements (07Nov00):

strontium
Symbol: Sr
Atomic number: 38
Atomic weight: 87.62
Soft yellowish metallic element, belongs to group 2 of the periodic
table. Highly reactive chemically. Sr-90 is present in radioactive
fallout
and has a half-life of 28 years. Discovered in 1798 by Klaproth and
Hope,
isolated in 1808 by Humphry Davy.