lanthanoid vs rare-earth element

lanthanoid and rare-earth element both are nouns.

Word	Noun	Adjective	Verb	Adverb
lanthanoid	Yes	No	No	No
rare-earth element	Yes	No	No	No

As nouns, lanthanoid and rare-earth element are synonyms defined as:

lanthanoid and rare-earth element: any element of the lanthanide series (atomic numbers 57 through 71)

Other synonyms of lanthanoid include lanthanide, lanthanon, rare earth.

lanthanoid (noun)	rare-earth element (noun)
any element of the lanthanide series (atomic numbers 57 through 71)	any element of the lanthanide series (atomic numbers 57 through 71)