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):

cannabis
    n 1: 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]
    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]