The Collaborative International Dictionary of English v.0.48:

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 developing a visible fruiting body
          with a stem and cap, usu. of the basidiomycetes;
          especially one of the genus Agaricus; a toadstool.
          Several species are edible; but many are very
          poisonous. The term mushroom is used most often for
          edible varieties, the poisonous ones being termed
          toadstools or other names. But this distinction is
          often ignored.
          [1913 Webster +PJC]

   2. One who rises suddenly from a low condition in life; an
      upstart. --Bacon.
      [1913 Webster]

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]

WordNet (r) 3.0 (2006):

Agaricus campestris
    n 1: common edible mushroom found naturally in moist open soil;
         the cultivated mushroom of commerce [syn: meadow
         mushroom, field mushroom, Agaricus campestris]