/see"zee euhm/ , n.
a rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Symbol: Cs; at. wt.: 132.905; at. no.: 55; sp. gr.: 1.9 at 20°C; melts at 28.5°C.
Also, caesium .
[ 1930-35; caesium, neut. of caesius bluish-grey; see -IUM ]