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]

WordNet (r) 3.0 (2006):

mushroom cloud
    n 1: 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]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "mushroom cloud":
   A-blast, A-bomb shelter, Cloudcuckooland, H-blast, Mach front,
   airborne radioactivity, alto-cumulus, alto-stratus, anvil cloud,
   atom blast, banner cloud, billowy cloud, blast wave, cap cloud,
   cirro-cumulus, cirro-fillum, cirro-nebula, cirro-stratus,
   cirro-velum, cirrus, cirrus cloud, cirrus haze, cirrus stripe,
   cloud, cloud band, cloud bank, cloud drift, cloud mass,
   cloud street, cloudland, cloudling, cloudscape, cocktail,
   cottony cloud, cumulo-cirro-stratus, cumulo-cirrus, cumulo-nimbus,
   cumulo-stratus, cumulus, cumulus cloud, curl cloud, dust cloud,
   fallout, fallout shelter, fission particles, flash burn,
   fleecy cloud, fractocumulus, ground zero, high fog, hydrogen blast,
   mackerel sky, mammatocumulus, nimbus, nimbus cloud,
   radioactive dust, rain cloud, scud, snail cloud, squall cloud,
   storm cloud, strato-cumulus, stratus, stratus cloud, thundercloud,
   thunderhead, water carrier, woolpack