The Collaborative International Dictionary of English v.0.48:

Henna \Hen"na\, n. [Ar. hinn[=a] alcanna (Lawsonia inermis
   syn. Lawsonia alba). Cf. Alcanna, Alkanet, Orchanet.]
   [1913 Webster]
   1. (Bot.) A thorny tree or shrub of the genus Lawsonia
      (Lawsonia alba). The fragrant white blossoms are used by
      the Buddhists in religious ceremonies. The powdered leaves
      furnish a red coloring matter used in the East to stain
      the nails and fingers, the manes of horses, etc.
      [1913 Webster]

   2. (Com.) The leaves of the henna plant, or a preparation or
      dyestuff made from them.
      [1913 Webster]