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]

The Elements (07Nov00):

caesium
Symbol: Cs
Atomic number: 55
Atomic weight: 132.90545
Soft silvery-white metallic element belonging to group 1 of the periodic
table. One of the three metals which are liquid at room temperature.
Cs-133
is the natural, and only stable, isotope. Fifteen other radioisotopes
exist.
Caesium reacts explosively with cold water, and ice at temperatures
above
157K. Caesium hydroxide is the strongest base known. Caesium is the most
electropositive, most alkaline and has the least ionization potential of
all the elements. Known uses include the basis of atomic clocks,
catalyst
for the hydrogenation of some organic compounds, and in photoelectric
cells.
Caesium was discovered by Gustav Kirchoff and Robert Bunsen in Germany
in
1860 spectroscopically. Its identification was based upon the bright
blue
lines in its spectrum. The name comes from the latin word caesius, which
means sky blue. Caesium should be considered highly toxic. Some of the
radioisotopes are even more toxic.