lanthanum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A soft, silvery-white, malleable, ductile, metallic rare-earth element, obtained chiefly from monazite and bastnaesite and used in glass manufacture and with other rare earths in carbon lights for movie and television studio lighting. Atomic number 57; atomic weight 138.91; melting point 920°C; boiling point 3,464°C; specific gravity 6.145 (at 25°C); valence 2, 3. cross-reference: Periodic Table.

from The Century Dictionary.

• noun Chemical symbol, La; atomic weight, 138. A rare metal discovered by Mosander in 1839–41, associated with didymium in the oxid of cerium, and so named from its properties having been previously concealed by those of cerium.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A metallic chemical element (symbol La) with an atomic number of 57.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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