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 (A. belladonna); the belladonna lily.