Source : Webster's Revised Unabridged Dictionary (1913)

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}, {bulbifera}, and {virosa}, and the
      {Conium maculatum}. See {Conium}.

   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}.

   2. (Bot.) An evergreen tree common in North America ({Abies,
      or Tsuga, Canadensis}); hemlock spruce.

            The murmuring pines and the hemlocks. --Longfellow.

   3. The wood or timber of the hemlock tree.

   {Ground hemlock}, or {Dwarf hemlock}. See under {Ground}.

Source : WordNet®

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: {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 tree}]