lanthanum

noun

1. a white soft metallic element that tarnishes readily; occurs in rare earth minerals and is usually classified as a rare earth

   noun.substance

   Synonyms atomic number 57 la

   Source: WordNet 3.0

2. 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.

   noun

   Source: webster1913