nicotine
    n 1: an alkaloid poison that occurs in tobacco; used in medicine
         and as an insecticide

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]