Source : Webster's Revised Unabridged Dictionary (1913)

Water hemlock \Wa"ter hem"lock\ (Bot)
   (a) A poisonous umbelliferous plant ({Cicuta virosa}) of
       Europe; also, any one of several plants of that genus.
   (b) A poisonous plant ({[OE]nanthe crocata}) resembling the
       above.

Source : WordNet®

water hemlock
     n : tall erect highly poisonous Eurasiatic perennial herb
         locally abundant in marshy areas [syn: {Cicuta verosa}]