The Collaborative International Dictionary of English v.0.48:

Nightshade \Night"shade`\, n. [AS. nichtscadu.] (Bot.)
   A common name of many species of the genus Solanum, given
   esp. to the Solanum nigrum, or black nightshade, a low,
   branching weed with small white flowers and black berries
   reputed to be poisonous.
   [1913 Webster]

   Deadly nightshade. Same as Belladonna
   (a) .

   Enchanter's nightshade. See under Enchanter.

   Stinking nightshade. See Henbane.

   Three-leaved nightshade. See Trillium.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

deadly \dead"ly\, a.
   1. Capable of causing death; mortal; fatal; destructive;
      certain or likely to cause death; as, a deadly blow or
      wound.
      [1913 Webster]

   2. Aiming or willing to destroy; implacable; desperately
      hostile; flagitious; as, deadly enemies.
      [1913 Webster]

            Thy assailant is quick, skillful, and deadly.
                                                  --Shak.
      [1913 Webster]

   3. Subject to death; mortal. [Obs.]
      [1913 Webster]

            The image of a deadly man.            --Wyclif (Rom.
                                                  i. 23).
      [1913 Webster]

   Deadly nightshade (Bot.), a poisonous plant; belladonna.
      See under Nightshade.
      [1913 Webster]

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):

deadly nightshade
    n 1: perennial Eurasian herb with reddish bell-shaped flowers
         and shining black berries; extensively grown in United
         States; roots and leaves yield atropine [syn: belladonna,
         belladonna plant, deadly nightshade, Atropa
         belladonna]
    2: poisonous perennial Old World vine having violet flowers and
       oval coral-red berries; widespread weed in North America
       [syn: bittersweet, bittersweet nightshade, climbing
       nightshade, deadly nightshade, poisonous nightshade,
       woody nightshade, Solanum dulcamara]