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Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   atomic number 69
       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]