The Collaborative International Dictionary of English v.0.48:

Marasmius \Marasmius\ prop. n.
   A genus of chiefly small white-spored mushrooms, including
   the Marasmius oreades, the mushroon that grow in a fairy
   ring.

   Syn: genus Marasmius.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Champignon \Cham*pi"gnon\, n. [F., a mushroom, ultimately fr. L.
   campus field. See Camp.] (Bot.)
   An edible species of mushroom (Agaricus campestris).
   [1913 Webster]

   Fairy ring champignon, the Marasmius oreades, which has a
      strong flavor but is edible.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fairy \Fair"y\, a.
   1. Of or pertaining to fairies.
      [1913 Webster]

   2. Given by fairies; as, fairy money. --Dryden.
      [1913 Webster]

   Fairy bird (Zool.), the Euoropean little tern (Sterna
      minuta); -- called also sea swallow, and hooded tern.


   Fairy bluebird. (Zool.) See under Bluebird.

   Fairy martin (Zool.), a European swallow (Hirrundo ariel)
      that builds flask-shaped nests of mud on overhanging
      cliffs.

   Fairy rings or Fairy circles, the circles formed in
      grassy lawns by certain fungi (as Marasmius Oreades),
      formerly supposed to be caused by fairies in their
      midnight dances; also, the mushrooms themselves. Such
      circles may have diameters larger than three meters.

   Fairy shrimp (Zool.), a European fresh-water phyllopod
      crustacean (Chirocephalus diaphanus); -- so called from
      its delicate colors, transparency, and graceful motions.
      The name is sometimes applied to similar American species.


   Fairy stone (Paleon.), an echinite.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Marasmius oreades
    n 1: mushroom that grows in a fairy ring [syn: fairy-ring
         mushroom, Marasmius oreades]