The Collaborative International Dictionary of English v.0.48:

Hemp \Hemp\ (h[e^]mp), n. [OE. hemp, AS. henep, h[ae]nep; akin
   to D. hennep, OHG. hanaf, G. hanf, Icel. hampr, Dan. hamp,
   Sw. hampa, L. cannabis, cannabum, Gr. ka`nnabis, ka`nnabos;
   cf. Russ. konoplia, Skr. [,c]a[.n]a; all prob. borrowed from
   some other language at an early time. Cf. Cannabine,
   Canvas.]
   1. (Bot.) A plant of the genus Cannabis (Cannabis
      sativa), the fibrous skin or bark of which is used for
      making cloth and cordage. The name is also applied to
      various other plants yielding fiber.
      [1913 Webster]

   2. The fiber of the skin or rind of the plant, prepared for
      spinning. The name has also been extended to various
      fibers resembling the true hemp.
      [1913 Webster]

   African hemp, Bowstring hemp. See under African, and
      Bowstring.

   Bastard hemp, the Asiatic herb Datisca cannabina.

   Canada hemp, a species of dogbane (Apocynum cannabinum),
      the fiber of which was used by the Indians.

   Hemp agrimony, a coarse, composite herb of Europe
      (Eupatorium cannabinum), much like the American boneset.


   Hemp nettle, a plant of the genus Galeopsis (Galeopsis
      Tetrahit), belonging to the Mint family.

   Indian hemp. See under Indian, a.

   Manila hemp, the fiber of Musa textilis.

   Sisal hemp, the fiber of Agave sisalana, of Mexico and
      Yucatan.

   Sunn hemp, a fiber obtained from a leguminous plant
      (Crotalaria juncea).

   Water hemp, an annual American weed (Acnida cannabina),
      related to the amaranth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Galeopsis tetrahit
    n 1: coarse bristly Eurasian plant with white or reddish flowers
         and foliage resembling that of a nettle; common as a weed
         in United States [syn: hemp nettle, dead nettle,
         Galeopsis tetrahit]