The Collaborative International Dictionary of English v.0.48:

Gadolinium \Gad`o*lin"i*um\, n. [NL. See Gadolinite.] (Chem.)
   A rare earth metallic element of the Lanthanide series, with
   a characteristic spectrum, found associated with yttrium and
   other rare earth elements. Symbol, Gd; it has an atomic
   number of 64, an atomic weight of 157.25 (C=12.011), and a
   valence of +3.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

gadolinium
    n 1: a ductile silvery-white ductile ferromagnetic trivalent
         metallic element of the rare earth group [syn:
         gadolinium, Gd, atomic number 64]

The Elements (07Nov00):

gadolinium
Symbol: Gd
Atomic number: 64
Atomic weight: 157.25
Soft silvery metallic element belonging to the lanthanoids. Seven
natural,
stable isotopes are known in addition to eleven artificial isotopes.
Gd-155 and Gd-157 and the best neutron absorbers of all elements.
Gadolinium compounds are used in electronics. Discovered by J.C.G
Marignac
in 1880.