The Collaborative International Dictionary of English v.0.48:

Henbane \Hen"bane`\, n. [Hen + bane.] (Bot.)
   A plant of the genus Hyoscyamus (Hyoscyamus niger). All
   parts of the plant are poisonous, and the leaves are used for
   the same purposes as belladonna. It is poisonous to domestic
   fowls; whence the name. Called also, stinking nightshade,
   from the fetid odor of the plant. See Hyoscyamus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

henbane
    n 1: poisonous fetid Old World herb having sticky hairy leaves
         and yellow-brown flowers; yields hyoscyamine and
         scopolamine [syn: henbane, black henbane, stinking
         nightshade, Hyoscyamus niger]