noun (n)

• a metallic element of the rare earth group; used in lasers; occurs in apatite and monazite and xenotime and ytterbite(noun.substance)

  Synonym:
  atomic number 65, tb
  

  source: wordnet30
• A rare metallic element, of uncertain identification, supposed to exist in certain minerals, as gadolinite and samarskite, with other rare ytterbium earth. Symbol Tr or Tb. Atomic weight 150.(noun)

  source: webster1913