The Collaborative International Dictionary of English v.0.48:

Fly amanita \Fly amanita\, Fly fungus \Fly fungus\ . (Bot.)
   A poisonous mushroom (Amanita muscaria, syn. Agaricus
   muscarius), having usually a bright red or yellowish cap
   covered with irregular white spots. It has a distinct volva
   at the base, generally an upper ring on the stalk, and white
   spores. Called also fly agaric, deadly amanita.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Amanita \Am`a*ni"ta\, n. [NL. See Amanitine.] (Bot.)
   A genus of poisonous fungi of the family Agaricace[ae],
   characterized by having a volva, an annulus, and white
   spores. The species resemble edible mushrooms, and are
   frequently mistaken for them. Amanita muscaria, syn.
   Agaricus muscarius, is the fly amanita, or fly agaric; and
   Agaricus phalloides is the death cup (or death cap).
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Amanita muscaria
    n 1: poisonous (but rarely fatal) woodland fungus having a
         scarlet cap with white warts and white gills [syn: fly
         agaric, Amanita muscaria]