from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Any of the abundant metallic elements of atomic number 57 through 71. Also called lanthanide.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. any element of the lanthanide series (atomic numbers 57 through 71)
So called because they were originally thought to be rare.(American Heritage® Dictionary of the English Language, Fourth Edition)