[ noun ] a rare polyvalent metallic element of the platinum group; it is found associated with platinum <noun.substance>
Ruthenium \Ru*the"ni*um\, n. [NL. So named from the Ruthenians, a Little Russian people, as coming from Russia, the metal having been found in the Ural mountains.] (Chem.) A rare element of the light platinum group, found associated with platinum ores, and isolated as a hard, brittle steel-gray metal which is very infusible. Symbol Ru. Atomic weight 103.5. Specific gravity 12.26. See {Platinum metals}, under {Platinum}.
Other members of the platinum group are ruthenium, iridium and osmium.