The Collaborative International Dictionary of English v.0.48:

Prickly ash \Prickly ash\ (Bot.),
   1. A prickly shrub (Xanthoxylum Americanum) with yellowish
      flowers appearing with the leaves; also called toothache
      tree. All parts of the plant are pungent and aromatic.
      The southern species is Xanthoxylum Carolinianum.
      --Gray.
      [1913 Webster]

   2. (Bot.), Hercules'-club, also called the Angelica tree.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Xanthoxylum \Xan*thox"y*lum\, n. [NL., from Gr. xanqo`s yellow +
   xy`lon wood.] (Bot.)
   A genus of prickly shrubs or small trees, the bark and rots
   of which are of a deep yellow color; prickly ash.
   [1913 Webster]

   Note: The commonest species in the Northern United States is
         Xanthoxylum Americanum. See Prickly ash, under
         Prickly.
         [1913 Webster]