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]

The Collaborative International Dictionary of English v.0.48:

Lawsonia \Law*so"ni*a\, n. (Bot.)
   An Asiatic and North African shrub (Lawsonia inermis), with
   smooth oval leaves, and fragrant white flowers. Henna is
   prepared from the leaves and twigs. In England the shrub is
   called Egyptian privet, and in the West Indies, Jamaica
   mignonette.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alcanna \Al*can"na\, n. [Sp. alcana, alhe?a, fr. Ar.
   al-hinn[=a]. See Henna, and cf. Alkanet.] (Bot.)
   An oriental shrub (Lawsonia inermis) from which henna is
   obtained.
   [1913 Webster]