lanthanide(n = noun.substance) lanthanoid, lanthanon, rare earth, rare-earth element - any element of the lanthanide series (atomic numbers 57 through 71);
is a kind of group, grouping
has particulars: terbium metal

   Any rare earth element of the lanthanide series (atomic numbers 57 through 71). [WordNet 1.5]

lanthanide, n. Chem. an element of the lanthanide series.

Idiom:
lanthanide series a series of 15 metallic elements from lanthanum to lutetium in the periodic table, having similar chemical properties: also called rare earths (see RARE(1)).

Etymology:
G Lanthanid (as LANTHANUM)

lanthanide series