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:

tetrahydrocannabinol \tet`ra*hy`dro*can*nab"i*nol\
   (t[e^]t`r[.a]*h[imac]`dr[-o]*k[a^]n*n[a^]b"[i^]n[add]l), n.
   (Chem. & Med.)
   A chemical substance (C21H20O2) which is produced by the
   hemp plant (Cannabis sativa), and is the physiologically
   active agent of dried preparations of that plant, called
   variously marijuana, hashish, ganja, hemp, etc.; also called
   THC. It causes the euphoric effect for which the
   preparations are smoked or chewed. It is used in medicine in
   a purified form as an antiemetic (an antinausea agent),
   especially in conjunction with chemotherapy of cancer. It
   occurs primarily as the [Delta]1-3,4-trans isomer, also
   called [Delta]9-THC, with small amounts of the
   [Delta]6-3,4-trans isomer detectable at about 1%. It is a
   controlled substance, classified as a hallucinogen, and its
   possession or distribution is illegal in almost all states of
   the United States. --MI11
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Cannabis \Can"na*bis\, prop. n. [L., hemp. See Canvas.]
   1. (Bot.) A genus of a single species belonging to the order
      Uricace[ae]; hemp.
      [1913 Webster]

   2. The Indian hemp plant plant Cannabis sativa syn.
      Cannabis Indica. See Cannabis Indica, below.
      [PJC]

   Cannabis Indica, the Indian hemp, now considered a variety
      of the common hemp (Cannabis sativa), originating in
      Asia but now grown widely around the world. It can be
      formed into a tough fiber that is used to make rope and
      tough fabric. The dried leaves and female flowers are
      smoked or chewed by people to produce a euphoric or
      hallucinogenic effect, caused mainly by the substance
      tetrahydrocannabinol (THC) contained in it. The
      THC-containing products of Cannabis are classified as a
      controlled substance, and posession and sale are illegal
      in most states of the United States. Once classified as a
      narcotic, the plant is now more commonly referred to in
      the category of euphoric or hallucinogenic agents.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

tetrahydrocannabinol
    n 1: psychoactive substance present in marijuana [syn:
         tetrahydrocannabinol, THC]