rüˈbidēəm noun
( -s )
Etymology: New Latin, from Latin rubidus red + New Latin -ium; from two red lines in its spectrum; akin to Latin ruber red
: a soft silvery metallic element of the alkali metal group that is found combined in small amounts in many minerals and mineral waters and the ashes of many plants usually accompanied by still smaller amounts of cesium, that is produced in metallic form by electrolysis or chemical reduction of its compounds, and that decomposes water with violence and inflames spontaneously in air — symbol Rb ; see element table