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]