Definition of CESIUM. : a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells â see element table.See cesium defined for kids.efinition of CESIUM. : a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells â see element table. See cesium defined for kids.