caesium > define
noun
- A soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal.
- synonyms: cesium, Cs, atomic number 55
by WordNet 3.0 © 2006 by Princeton University
(n.) 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.
by Webster's Revised Unabridged Dictionary