The Collaborative International Dictionary of English v.0.48:

hafnium \haf"ni*um\ (h[a^]f"n[=e]*[u^]m or h[aum]f"n[=e]*[u^]m),
   n.
   A metallic element of atomic number 72 present together with
   zirconium to the extent of 1% to 5% in zirconium minerals. It
   is a poisonous, ductile metal with a brilliant silver luster,
   has an atomic weight of 178.49, and has a high melting point
   (2227[deg] C). It is used in nuclear reactors, and
   incandescent lamps as a scavenger of oxygen and nitrogen. See
   also norium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hafnium
    n 1: a grey tetravalent metallic element that resembles
         zirconium chemically and is found in zirconium minerals;
         used in filaments for its ready emission of electrons [syn:
         hafnium, Hf, atomic number 72]

The Elements (07Nov00):

hafnium
Symbol: Hf
Atomic number: 72
Atomic weight: 178.49
Silvery lustrous metallic transition element. Used in tungsten alloys in
filaments and electrodes, also acts as a neutron absorber. First
reported
by Urbain in 1911, existence was finally established in 1923 by D.
Coster,
G.C. de Hevesy in 1923.