The Collaborative International Dictionary of English v.0.48:

Molybdenum \Mol`yb*de"num\, n. [NL.: cf. F. molybd[`e]ne. See
   Molybdena.] (Chem.)
   A rare element of the chromium group, occurring in nature in
   the minerals molybdenite and wulfenite, and when reduced
   obtained as a hard, silver-white, difficulty fusible metal.
   Symbol Mo. Atomic number 42. Atomic weight 95.94.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

molybdenum
    n 1: a polyvalent metallic element that resembles chromium and
         tungsten in its properties; used to strengthen and harden
         steel [syn: molybdenum, Mo, atomic number 42]