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.-any large fungus, especially one of the genus agaricus; a toadstool. several species are edible; but many are very poisonous.-one who rises suddenly from a low condition in life; an upstart.-of or pertaining to mushrooms; as, mushroom catchup.-resembling mushrooms in rapidity of growth and shortness of duration; short-lived; ephemerial; as, mushroom cities.