Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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