Source : Webster's Revised Unabridged Dictionary (1913)

Caesium \C[ae]"si*um\, 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 weight 132.6.

Source : WordNet®

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