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
    n 1: genus of widely distributed agarics that have white spores
         and are poisonous with few exceptions [syn: Amanita,
         genus Amanita]