Source : Webster's Revised Unabridged Dictionary (1913)

Mushroom \Mush"room\, n. [OE. muscheron, OF. mouscheron, F.
   mousseron; perhaps fr. mousse moss, of German origin. See
   {Moss}.]
   1. (Bot.)
      (a) An edible fungus ({Agaricus campestris}), having a
          white stalk which bears a convex or oven flattish
          expanded portion called the pileus. This is whitish
          and silky or somewhat scaly above, and bears on the
          under side radiating gills which are at first
          flesh-colored, but gradually become brown. The plant
          grows in rich pastures and is proverbial for rapidity
          of growth and shortness of duration. It has a pleasant
          smell, and is largely used as food. It is also
          cultivated from spawn.
      (b) Any large fungus, especially one of the genus
          {Agaricus}; a toadstool. Several species are edible;
          but many are very poisonous.

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

   {Fairy ring champignon}, the {Marasmius oreades}, which has a
      strong flavor but is edible.

Source : WordNet®

Agaricus campestris
     n : common edible mushroom found naturally in moist open soil;
         the cultivated mushroom of commerce [syn: {meadow
         mushroom}, {field mushroom}]