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 ]