fairy ring vs annulus

fairy ring and annulus both are nouns.

Word	Noun	Adjective	Verb	Adverb
fairy ring	Yes	No	No	No
annulus	Yes	No	No	No

As nouns, annulus is a hypernym of fairy ring; that is, annulus is a word with a broader meaning than fairy ring:

fairy ring: a ring of fungi marking the periphery of the perennial underground growth of the mycelium
annulus: a toroidal shape

Other hypernyms of fairy ring include anchor ring, doughnut, halo, ring.

fairy ring (noun)	annulus (noun)
a ring of fungi marking the periphery of the perennial underground growth of the mycelium	(Fungi) a remnant of the partial veil that in mature mushrooms surrounds the stem like a collar
	a toroidal shape