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]

WordNet (r) 3.0 (2006):

nightshade
    n 1: any of numerous shrubs or herbs or vines of the genus
         Solanum; most are poisonous though many bear edible fruit