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1.                 From 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]