The Collaborative International Dictionary of English v.0.48:

Terbium \Ter"bi*um\, n. [NL., fr. Ytterby, in Sweden. See
   Erbium.] (Chem.)
   A rare metallic element, found in certain minerals, as
   gadolinite and samarskite, with other rare earths such as
   ytterbium. Symbol Tb. Atomic number 65. Atomic weight
   158.925.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

terbium
    n 1: a metallic element of the rare earth group; used in lasers;
         occurs in apatite and monazite and xenotime and ytterbite
         [syn: terbium, Tb, atomic number 65]

The Elements (07Nov00):

terbium
Symbol: Tb
Atomic number: 65
Atomic weight: 158.924
Silvery metallic element belonging to the lanthanoids. Tb-159 is the
only
stable isotope, there are seventeen artificial isotopes. Discovered by
G.G. Mosander in 1843.