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]

WordNet (r) 3.0 (2006):

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