The Collaborative International Dictionary of English v.0.48:

cesium \cesium\ n.
   the chemical element of atomic number 55. It is a univalent
   element, the most electropositive metal. Symbol Cs; atomic
   weight 132.905. IT has a melting point of 28.4[deg] C.

   Syn: caesium, Cs.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

cesium
    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]