The Collaborative International Dictionary of English v.0.48:

Nicotine \Nic"o*tine\ (? or ?), n. [F. nicotine. See
   Nicotian.] (Chem.)
   An alkaloid which is the active principle of tobacco
   (C10H14N2). It occurs in tobacco plants (Nicotiana
   tabacum and Nicotiana rusticum) to the extent of 2 to 8%,
   in combination with malic acid or citric acid. It is a
   colorless, transparent, oily liquid, having an acrid odor,
   and an acrid burning taste. It is intensely poisonous. The
   apparently addictive effects of tobacco smoking have been
   ascribed largely to the effect of nicotine, and the
   controlled administration of nicotine on various forms has
   been used as a technique for assisting efforts to stop the
   smoking habit. --Ure.
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Tobacco \To*bac"co\, n. [Sp. tabaco, fr. the Indian tabaco the
   tube or pipe in which the Indians or Caribbees smoked this
   plant. Some derive the word from Tabaco, a province of
   Yucatan, where it was said to be first found by the
   Spaniards; others from the island of Tobago, one of the
   Caribbees. But these derivations are very doubtful.]
   1. (Bot.) An American plant (Nicotiana Tabacum) of the
      Nightshade family, much used for smoking and chewing, and
      as snuff. As a medicine, it is narcotic, emetic, and
      cathartic. Tobacco has a strong, peculiar smell, and an
      acrid taste.
      [1913 Webster]

   Note: The name is extended to other species of the genus, and
         to some unrelated plants, as Indian tobacco (Nicotiana
         rustica, and also Lobelia inflata), mountain tobacco
         (Arnica montana), and Shiraz tobacco (Nicotiana
         Persica).
         [1913 Webster]

   2. The leaves of the plant prepared for smoking, chewing,
      etc., by being dried, cured, and manufactured in various
      ways.
      [1913 Webster]

   Tobacco box (Zool.), the common American skate.

   Tobacco camphor. (Chem.) See Nicotianine.

   Tobacco man, a tobacconist. [R.]

   Tobacco pipe.
      (a) A pipe used for smoking, made of baked clay, wood, or
          other material.
      (b) (Bot.) Same as Indian pipe, under Indian.

   Tobacco-pipe clay (Min.), a species of clay used in making
      tobacco pipes; -- called also cimolite.

   Tobacco-pipe fish. (Zool.) See Pipemouth.

   Tobacco stopper, a small plug for pressing down the tobacco
      in a pipe as it is smoked.

   Tobacco worm (Zool.), the larva of a large hawk moth
      (Sphinx Carolina syn. Phlegethontius Carolina). It is
      dark green, with seven oblique white stripes bordered
      above with dark brown on each side of the body. It feeds
      upon the leaves of tobacco and tomato plants, and is often
      very injurious to the tobacco crop. See Illust. of Hawk
      moth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Nicotiana tabacum
    n 1: tall erect South American herb with large ovate leaves and
         terminal clusters of tubular white or pink flowers;
         cultivated for its leaves [syn: common tobacco,
         Nicotiana tabacum]