The Collaborative International Dictionary of English v.0.48:

Erbium \Er"bi*um\, n. [NL. from Ytterby, in Sweden, where
   gadolinite is found. Cf. Terbium, Yttrium, Ytterbium.]
   (Chem.)
   A rare earth element of the lanthanide series associated with
   several other rare elements in the mineral gadolinite from
   Ytterby in Sweden. Symbol Er. It has atomic number 68 and an
   atomic weight of 167.26. The pure element is metallic with a
   bright, silvery luster. It is relatively stable in air, not
   oxidizing as quickly as some other rare earths. Its salts are
   rose-colored and give characteristic spectra, and the pink
   oxide has been added as a colorant in glass and porcelain
   enamel glazes. Its sesquioxide Er2O3 is called erbia.
   --HCP61
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

erbium
    n 1: a trivalent metallic element of the rare earth group;
         occurs with yttrium [syn: erbium, Er, atomic number
         68]

The Elements (07Nov00):

erbium
Symbol: Er
Atomic number: 68
Atomic weight: 167.26
Soft silvery metallic element which belongs to the lanthanoids. Six
natural isotopes that are stable. Twelve artificial isotopes are known.
Used in nuclear technology as a neutron absorber. It is being
investigated
for other possible uses. Discovered by Carl G. Mosander in 1843.