The Collaborative International Dictionary of English v.0.48:

Thulium \Thu"li*um\, n. [NL. See Thule.] (Chem.)
   A rare metallic element of the lanthanide group having atomic
   number 69, and atomic weight 168.93. It is found in the
   mineral gadolinite and other minerals, together with other
   rare earths. For more information see the data from
   ChemGlobe.
   [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

thulium
    n 1: a soft silvery metallic element of the rare earth group;
         isotope 170 emits X-rays and is used in small portable
         X-ray machines; it occurs in monazite and apatite and
         xenotime [syn: thulium, Tm, atomic number 69]