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: Webster's Revised Unabridged Dictionary (1913)