The Collaborative International Dictionary of English v.0.48:

Wormwood \Worm"wood\, n. [AS. werm?d, akin to OHG. wermuota,
   wormuota, G. wermuth, wermut; of uncertain origin.]
   [1913 Webster]
   1. (Bot.) A composite plant (Artemisia Absinthium), having
      a bitter and slightly aromatic taste, formerly used as a
      tonic and a vermifuge, and to protect woolen garments from
      moths. It gives the peculiar flavor to the cordial called
      absinthe. The volatile oil is a narcotic poison. The term
      is often extended to other species of the same genus.
      [1913 Webster]

   2. Anything very bitter or grievous; bitterness.
      [1913 Webster]

            Lest there should be among you a root that beareth
            gall and wormwood.                    --Deut. xxix.
                                                  18.
      [1913 Webster]

   Roman wormwood (Bot.), an American weed (Ambrosia
      artemisiaefolia); hogweed.

   Tree wormwood (Bot.), a species of Artemisia (probably
      Artemisia variabilis) with woody stems.

   Wormwood hare (Zool.), a variety of the common hare (Lepus
      timidus); -- so named from its color.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wormwood
    n 1: any of several low composite herbs of the genera Artemisia
         or Seriphidium

Easton's 1897 Bible Dictionary:

Wormwood
   Heb. la'anah, the Artemisia absinthium of botanists. It is noted
   for its intense bitterness (Deut. 29:18; Prov. 5:4; Jer. 9:15;
   Amos 5:7). It is a type of bitterness, affliction, remorse,
   punitive suffering. In Amos 6:12 this Hebrew word is rendered
   "hemlock" (R.V., "wormwood"). In the symbolical language of the
   Apocalypse (Rev. 8:10, 11) a star is represented as falling on
   the waters of the earth, causing the third part of the water to
   turn wormwood.

     The name by which the Greeks designated it, absinthion, means
   "undrinkable." The absinthe of France is distilled from a
   species of this plant. The "southernwood" or "old man,"
   cultivated in cottage gardens on account of its fragrance, is
   another species of it.