The Collaborative International Dictionary of English v.0.48:

Toadstool \Toad"stool`\, n. (Bot.)
   A name given to many umbrella-shaped fungi, mostly of the
   genus Agaricus. The species are almost numberless. They
   grow on decaying organic matter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toadstool
    n 1: common name for an inedible or poisonous agaric
         (contrasting with the edible mushroom) [ant: mushroom]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "toadstool":
   algae, autophyte, bean, bracken, brown algae, climber, conferva,
   confervoid, creeper, diatom, fern, fruits and vegetables, fucus,
   fungus, grapevine, green algae, gulfweed, herb, heterophyte, ivy,
   kelp, legume, lentil, liana, lichen, liverwort, mold, moss,
   mushroom, parasite, parasitic plant, pea, perthophyte,
   phytoplankton, planktonic algae, plant families, puffball, pulse,
   red algae, rockweed, rust, saprophyte, sargasso, sargassum,
   sea lentil, sea moss, sea wrack, seaweed, smut, succulent, vetch,
   vine, wort, wrack