Source : Webster's Revised Unabridged Dictionary (1913)

Yttrium \Yt"tri*um\, n. [NL., from Ytterby, in Sweden. See
   {Erbium}.] (Chem.)
   A rare metallic element of the boron-aluminium group, found
   in gadolinite and other rare minerals, and extracted as a
   dark gray powder. Symbol Y. Atomic weight, 89. [Written also
   {ittrium}.]

   Note: Associated with yttrium are certain rare elements, as
         erbium, ytterbium, samarium, etc., which are separated
         in a pure state with great difficulty. They are studied
         by means of their spark or phosphorescent spectra.
         Yttrium is now regarded as probably not a simple
         element, but as a mixture of several substances.

Source : WordNet®

yttrium
     n : a silvery metallic element that is common in rare-earth
         minerals; used in magnesium and aluminum alloys [syn: {Y},
          {atomic number 39}]