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 coral
    n 1: flattened disk-shaped stony coral (usually solitary and
         unattached)