The Collaborative International Dictionary of English v.0.48:

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 number 39. Atomic weight,
   88.9. [Written also ittrium.]
   [1913 Webster +PJC]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

yttrium
    n 1: a silvery metallic element that is common in rare-earth
         minerals; used in magnesium and aluminum alloys [syn:
         yttrium, Y, atomic number 39]