Source : Webster's Revised Unabridged Dictionary (1913)

Lanthanum \Lan"tha*num\, n. [NL., fr. Gr. ? to lie hid, to be
   concealed.] (Chem.)
   A rare element of the group of the earth metals, allied to
   aluminium. It occurs in certain rare minerals, as cerite,
   gadolinite, orthite, etc., and was so named from the
   difficulty of separating it from cerium, didymium, and other
   rare elements with which it is usually associated. Atomic
   weight 138.5. Symbol La. [Formerly written also
   {lanthanium}.]

Source : WordNet®

lanthanum
     n : a white soft metallic element that tarnishes readily; occurs
         in rare earth minerals and is usually classified as a
         rare earth [syn: {La}, {atomic number 57}]