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®

Oenanthe crocata
     n : European poisonous herb having tuberous roots, yellow juice
         that stains the skin, yellow flowers and foliage
         resembling celery; all parts extremely poisonous [syn: {water
         dropwort}, {hemlock water dropwort}]