noun
also cae·si·um ˈsēzēəm sometimes ˈsēzhē- or ˈsēsē- or ˈkīsē-
( -s )
Etymology: New Latin, from Latin caesius bluish gray + New Latin -ium; from two blue lines in its spectrum — more at caesious
: a silver-white soft ductile metallic element of the alkali metal group that is the most electropositive element known, found usually with rubidium and lithium (as in pollucite), and used especially in the form of its compounds and alloys in electron tubes and photoelectric cells — symbol Cs ; see element table