water hemlock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several poisonous perennial herbs of the genus Cicuta of the parsley family, especially C. maculata, of marshy areas in eastern and central North America, having compound umbels of small white flowers and bipinnately compound leaves.

from The Century Dictionary.

• noun See Cicuta.
• noun The hemlock water-dropwort, Œnanthe crocata, otherwise called dead-tongue; also Œ. Phellandrium, distinguished as fine-leafed water-hemlock.

from the GNU version of the Collaborative International Dictionary of English.

• A poisonous umbelliferous plant (Cicuta virosa) of Europe; also, any one of several plants of that genus.
• A poisonous plant (Œnanthe crocata) resembling the above.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun tall erect highly poisonous Eurasiatic perennial herb locally abundant in marshy areas