The Collaborative International Dictionary of English v.0.48:

Hemlock \Hem"lock\, n. [OE. hemeluc, humloc, AS. hemlic,
   hymlic.]
   1. (Bot.) The name of several poisonous umbelliferous herbs
      having finely cut leaves and small white flowers, as the
      Cicuta maculata, Cicuta bulbifera, and Cicuta
      virosa, and the Conium maculatum. See Conium.
      [1913 Webster]

   Note: The potion of hemlock administered to Socrates is by
         some thought to have been a decoction of Cicuta
         virosa, or water hemlock, by others, of Conium
         maculatum.
         [1913 Webster]

   2. (Bot.) An evergreen tree common in North America (Abies
      Canadensis or Tsuga Canadensis); hemlock spruce.
      [1913 Webster]

            The murmuring pines and the hemlocks. --Longfellow.
      [1913 Webster]

   3. The wood or timber of the hemlock tree.
      [1913 Webster]

   Ground hemlock, or Dwarf hemlock. See under Ground.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hemlock
    n 1: poisonous drug derived from an Eurasian plant of the genus
         Conium; "Socrates refused to flee and died by drinking
         hemlock"
    2: large branching biennial herb native to Eurasia and Africa
       and adventive in North America having large fernlike leaves
       and white flowers; usually found in damp habitats; all parts
       extremely poisonous [syn: hemlock, poison hemlock,
       poison parsley, California fern, Nebraska fern, winter
       fern, Conium maculatum]
    3: soft coarse splintery wood of a hemlock tree especially the
       western hemlock
    4: an evergreen tree [syn: hemlock, hemlock tree]