Source : Webster's Revised Unabridged Dictionary (1913)

Henna \Hen"na\, n. [Ar. hinn[=a] alcanna ({Lawsonia inermis or
   alba}). Cf. {Alcanna}, {Alkanet}, {Orchanet}.]
   1. (Bot.) A thorny tree or shrub of the genus {Lawsonia} ({L.
      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 hails and fingers, the manes of horses, etc.

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

Source : WordNet®

henna
     n : a reddish brown dye used especially on hair
     v : apply henna to one's hair; "She hennas her hair every month"