Real Dictionary

fairy ring in American English
a circle of mushrooms often seen on grassy ground, originating from one initial mycelium and often accompanied by a circle of darker grass
noun: any of numerous mushrooms of meadows and open woods, esp. the well-known Marasmius oreades, that spread in rings originating from mycelial growth: formerly supposed to mark the paths laid by dancing fairies

fairy ring in British English
noun: a ring of dark luxuriant vegetation in grassy ground corresponding to the edge of an underground fungal mycelium: popularly associated with the dancing of fairies: seasonally marked by a ring of mushrooms