The Collaborative International Dictionary of English v.0.48:

Belladonna \Bel`la*don"na\, n. [It., literally fine lady; bella
   beautiful + donna lady.] (Bot.)
   (a) An herbaceous European plant (Atropa belladonna) with
       reddish bell-shaped flowers and shining black berries.
       The whole plant and its fruit are very poisonous, and the
       root and leaves are used as powerful medicinal agents.
       Its properties are largely due to the alkaloid atropine
       which it contains. Called also deadly nightshade.
   (b) A species of Amaryllis (Amaryllis belladonna); the
       belladonna lily.
       [1913 Webster]

WordNet (r) 3.0 (2006):

Amaryllis belladonna
    n 1: amaryllis of South Africa often cultivated for its fragrant
         white or rose flowers [syn: belladonna lily, naked
         lady, Amaryllis belladonna]