The American Heritage® Dictionary of the English Language, 4th Edition
n. A soft, silvery-gray metallic rare-earth element, used in x-ray and color television tubes. Atomic number 65; atomic weight 158.925; melting point 1,356°C; boiling point 3,123°C; specific gravity 8.229; valence 3, 4. See Table at element.
n. a metallic chemical element (symbol Tb) with an atomic number of 65
the GNU version of the Collaborative International Dictionary of English
n. A rare metallic element, found in certain minerals, as gadolinite and samarskite, with other rare earths such as ytterbium. Symbol Tb. Atomic number 65. Atomic weight 158.925.
The Century Dictionary and Cyclopedia
n. A rare element, not yet isolated, occurring in the samarskite of North Carolina and certain other rare minerals, associated with erbium and yttrium.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a metallic element of the rare earth group; used in lasers; occurs in apatite and monazite and xenotime and ytterbite
Word Usage
"Indeed, some international prices haven't fallen at all, in particular those that are known as heavy rare earths, like terbium, which is used in advanced lasers and optics."