The Collaborative International Dictionary of English v.0.48:

Southernwood \South"ern*wood`\, n. (Bot.)
   A shrubby species of wormwood (Artemisia Abrotanum) having
   aromatic foliage. It is sometimes used in making beer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Artemisia abrotanum
    n 1: shrubby European wormwood naturalized in North America;
         sometimes used in brewing beer [syn: southernwood,
         Artemisia abrotanum]