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Definition of lanthanum:

  1. 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.


atomic number 57, lah, La.