The Collaborative International Dictionary of English v.0.48:

Lanthanum \Lan"tha*num\ (l[a^]n"th[.a]*n[u^]m), n. [NL., fr. Gr.
   lanqa`nein to lie hid, to be concealed.] (Chem.)
   A rare element of the rare earth group of the metals, of
   atomic number 57, allied to aluminum. 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 earth elements with which it is
   usually associated. Atomic weight 138.9. Symbol La. [Formerly
   written also lanthanium.]
   [1913 Webster]