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]

WordNet (r) 3.0 (2006):

lanthanum
    n 1: a white soft metallic element that tarnishes readily;
         occurs in rare earth minerals and is usually classified as
         a rare earth [syn: lanthanum, La, atomic number 57]

The Elements (07Nov00):

lanthanum
Symbol: La
Atomic number: 57
Atomic weight: 138.9055
(From the Greek word lanthanein, to lie hidden) Silvery metallic element
belonging to group 3 of the periodic table and oft considered to be one
of
the lanthanoids. Found in some rare-earth minerals. Twenty-five natural
isotopes exist. La-139 which is stable, and La-138 which has a half-life
of 10^10 to 10^15 years. The other twenty-three isotopes are
radioactive.
It resembles the lanthanoids chemically. Lanthanum has a low to moderate
level of toxicity, and should be handled with care. Discovered in 1839
by
C.G. Mosander.