The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ruthenium
    n 1: a rare polyvalent metallic element of the platinum group;
         it is found associated with platinum [syn: ruthenium,
         Ru, atomic number 44]

The Elements (07Nov00):

ruthenium
Symbol: Ru
Atomic number: 44
Atomic weight: 101.07
Hard white metallic transition element. Found with platinum, used as a
catalyst in some platinum alloys. Dissolves in fused alkalis, and is not
attacked by acids. Reacts with halogens and oxygen at high temperatures.
Isolated in 1844 by K.K. Klaus.