Merriam-Webster Collegiate Dictionary - fairy ring

 noun  Etymology: from the folk belief that such rings were dancing places of the fairies  Date: 1599  1. a ring of basidiomycetous mushrooms produced at the periphery of a body of mycelium which has grown outward from an initial growth point; also a ring of luxuriant vegetation especially when associated with these mushrooms  2. a mushroom (especially Marasmius oreades) that commonly grows in ~s