The Collaborative International Dictionary of English v.0.48:

lanthanide \lanthanide\ n.
   Any rare earth element of the lanthanide series (atomic
   numbers 57 through 71).

   Syn: rare earth, rare-earth element, lanthanon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lanthanide
    n 1: any element of the lanthanide series (atomic numbers 57
         through 71) [syn: rare earth, rare-earth element,
         lanthanoid, lanthanide, lanthanon]