The Collaborative International Dictionary of English v.0.48:

caesium \caesium\, Caesium \C[ae]"si*um\(s[=e]"z[-e]*[u^]m), n.
   [NL., from L. caesius bluish gray.] (Chem.)
   A rare alkaline metal found in mineral water; -- so called
   from the two characteristic blue lines in its spectrum. It
   was the first element discovered by spectrum analysis, and is
   the most strongly basic and electro-positive substance known.
   Symbol Cs. Atomic number 55. Atomic weight 132.6.

   Syn: cesium, Cs
        [1913 Webster]

WordNet (r) 3.0 (2006):

caesium
    n 1: a soft silver-white ductile metallic element (liquid at
         normal temperatures); the most electropositive and alkaline
         metal [syn: cesium, caesium, Cs, atomic number 55]