noun Any of the 15 rare earth elements from lanthanum to lutetium in the periodic table; because their outermost orbitals are not filled, they have very similar chemistry; below them are the actinides.
noun Any of the 14 rare earth elements from lanthanum to ytterbium (lutetium is excluded by some authors as it is a d-block rather than an f-block element).
noun Any of the 14 rare earth elements from cerium to lutetium (lanthanum having been excluded because it was thought to be a d-block element in the past).