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]

WordNet (r) 3.0 (2006):

marihuana
    n 1: a strong-smelling plant from whose dried leaves a number of
         euphoriant and hallucinogenic drugs are prepared [syn:
         marijuana, marihuana, ganja, Cannabis sativa]
    2: the most commonly used illicit drug; considered a soft drug,
       it consists of the dried leaves of the hemp plant; smoked or
       chewed for euphoric effect [syn: cannabis, marijuana,
       marihuana, ganja]