The Collaborative International Dictionary of English v.0.48:

marijuana \marijuana\ n.
   1. A strong-smelling Asian plant (Cannabis sativa), also
      called hemp, from which a number of euphorogenic and
      halucinogenic drugs are prepared. The euphoric effect is
      predominently due to tetrahydrocannabinol (THC). [Also
      spelled marihuana.]

   Syn: cannabis, ganja, pot, grass, marihuana, Cannabis
        sativa.
        [WordNet 1.5]

   2. The dried leaves or the female flowers of the hemp plant,
      which is smoked or chewed to obtain a euphoric effect. The
      flowers usually have a higher concentration of the active
      ingredient, tetrahydrocannabinol.

   Syn: cannabis, ganja, pot, grass, marihuana, dope, weed,
        gage, sess, sens, smoke, skunk, Mary Jane.
        [WordNet 1.5]

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):

hemp
    n 1: a plant fiber
    2: any plant of the genus Cannabis; a coarse bushy annual with
       palmate leaves and clusters of small green flowers; yields
       tough fibers and narcotic drugs [syn: cannabis, hemp]
    3: a rope that is used by a hangman to execute persons who have
       been condemned to death by hanging [syn: hangman's rope,
       hangman's halter, halter, hemp, hempen necktie]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "hemp":
   DET, DMT, LSD, Mary Jane, STP, THC, acid, antidepressant,
   ataractic, ax, block, cross, death chair, death chamber,
   diethyltryptamine, dimethyltryptamine, drop, electric chair, gage,
   gallows, gallows-tree, ganja, gas chamber, gibbet, grass,
   guillotine, hallucinogen, halter, hash, hashish, hay,
   hempen collar, hot seat, joint, kava, lethal chamber, maiden,
   marijuana, mescal, mescal bean, mescal button, mescaline,
   mind-altering drug, mind-blowing drug, mind-expanding drug,
   morning glory seeds, noose, peyote, pot, psilocin, psilocybin,
   psychedelic, psychic energizer, psychoactive drug, psychochemical,
   psychotomimetic, reefer, roach, rope, scaffold, stake, stick, tea,
   the chair, tranquilizer, tree, weed



The Devil's Dictionary (1881-1906):

HEMP, n.  A plant from whose fibrous bark is made an article of
neckwear which is frequently put on after public speaking in the open
air and prevents the wearer from taking cold.