The Collaborative International Dictionary of English v.0.48:

Ytterbium \Yt*ter"bi*um\, n. [NL., fr. Ytterby, in Sweden. See
   Erbium.] (Chem.)
   A rare element of the boron group, sometimes associated with
   yttrium or other related elements, as in euxenite and
   gadolinite. Symbol Yb; provisional atomic weight 173.2. Cf.
   Yttrium.
   [1913 Webster]

   Note: Ytterbium is associated with other rare elements, and
         probably has not been prepared in a pure state.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ytterbium
    n 1: a soft silvery metallic element; a rare earth of the
         lanthanide series; it occurs in gadolinite and monazite and
         xenotime [syn: ytterbium, Yb, atomic number 70]