The Collaborative International Dictionary of English v.0.48:

Mushroom \Mush"room\, a.
   1. Of or pertaining to mushrooms; as, mushroom catchup.
      [1913 Webster]

   2. Resembling mushrooms in rapidity of growth and shortness
      of duration; short-lived; ephemerial; as, mushroom cities.
      [1913 Webster]

   Mushroom anchor, an anchor shaped like a mushroom, capable
      of grasping the ground in whatever way it falls.

   Mushroom coral (Zool.), any coral of the genus Fungia.
      See Fungia.

   Mushroom spawn (Bot.), the mycelium, or primary filamentous
      growth, of the mushroom; also, cakes of earth and manure
      containing this growth, which are used for propagation of
      the mushroom.

   mushroom cloud, a cloud of smoke rising and then spreading
      laterally to take on the shape of a mushroom -- caused by
      large fires or explosions, esp. nuclear explosions.
      [1913 Webster +PJC]

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:

mushroom \mush"room\, v. i.
   1. to grow or expand rapidly.
      [PJC]

   2. to grow so much and so rapidly as to change qualitatively;
      used with into; as, a minor border skirmish mushroomed
      into a full-blown war.
      [PJC]

WordNet (r) 3.0 (2006):

mushroom
    n 1: common name for an edible agaric (contrasting with the
         inedible toadstool) [ant: toadstool]
    2: mushrooms and related fleshy fungi (including toadstools,
       puffballs, morels, coral fungi, etc.)
    3: any of various fleshy fungi of the subdivision Basidiomycota
       consisting of a cap at the end of a stem arising from an
       underground mycelium
    4: a large cloud of rubble and dust shaped like a mushroom and
       rising into the sky after an explosion (especially of a
       nuclear bomb) [syn: mushroom, mushroom cloud, mushroom-
       shaped cloud]
    5: fleshy body of any of numerous edible fungi
    v 1: pick or gather mushrooms; "We went mushrooming in the Fall"
    2: grow and spread fast; "The problem mushroomed"

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "mushroom":
   Ectocarpales, Phaeophyceae, algae, autophyte, ball, balloon, bead,
   bean, blossom, blow up, bracken, brew, brown algae, burgeon, burst,
   climber, conferva, confervoid, conglobulate, creeper, detonate,
   develop, diatom, fern, flourish, fruits and vegetables, fucus,
   fungi, fungus, gather, germinate, globe, go off, grapevine,
   green algae, grow, grow up, gulfweed, herb, heterophyte,
   hypertrophy, increase, ivy, kelp, legume, lentil, liana, lichen,
   liverwort, mature, mold, molds, moss, newly-rich, nouveau-riche,
   outgrow, overdevelop, overgrow, overtop, parasite, parasitic plant,
   parvenu, pea, perthophyte, phytoplankton, planktonic algae,
   plant families, procreate, puffball, pullulate, pulse, red algae,
   reproduce, rockweed, rust, saprophyte, sargasso, sargassum,
   sea lentil, sea moss, sea wrack, seaweed, shoot up, smut, snowball,
   sphere, spherify, spring up, sprout, sprout up, succulent,
   thallogens, thrive, toadstool, tower, upshoot, upspear, upspring,
   upsprout, upstart, vegetate, vetch, vine, wax, wort, wrack