ytterbium

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

• noun A soft, bright, silvery rare-earth element occurring in three allotropic forms and used as an x-ray source for portable irradiation devices, as a dopant for laser materials, and in solar cells and some special alloys. Atomic number 70; atomic weight 173.05; melting point 824°C; boiling point 1,196°C; specific gravity 6.903 (alpha form), 6.966 (beta form); valence 2, 3. cross-reference: Periodic Table.

from The Century Dictionary.

• noun In making the ‘glowers’ or filaments for the Nernst incandescent electric lamps it has been found that the mixture of zirconia with the earths of the yttria group gives the most satisfactory results when the latter contain a large proportion of ytterbium as compared with yttrium.
• noun Chemical symbol, Yb; atomic weight, 173 (?). An element discovered by Marignac in gadolinite, in regard to which little is known.

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

• noun (Chem.) A rare element of the boron group, sometimes associated with yttrium or other related elements, as in euxenite and gadolinite. Symbol Yb; provisional atomic weight 173.2. Cf. yttrium.

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

• noun A metallic chemical element (symbol Yb) with an atomic number of 70.

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

• noun a soft silvery metallic element; a rare earth of the lanthanide series; it occurs in gadolinite and monazite and xenotime