Mush·room n.
 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.
 2. One who rises suddenly from a low condition in life; an upstart.


 Cham·pi·gnon n.  Bot. An edible species of mushroom (Agaricus campestris).
 Fairy ring champignon, the Marasmius oreades, which has a strong flavor but is edible.

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