The Collaborative International Dictionary of English v.0.48:

Moxa \Mox"a\, n. [A corruption of Japan. mogusa (pronounced
   mongsa), an escharotic made from the plant yomigi: cf. F.
   moxa.]
   1. (Med.) A soft woolly mass prepared from the young leaves
      of Artemisia Chinensis, and used as a cautery by burning
      it on the skin; hence, any substance used in a like
      manner, as cotton impregnated with niter, amadou.
      [1913 Webster]

   2. (Bot.) A plant from which this substance is obtained, esp.
      Artemisia Chinensis, and Artemisia moxa.
      [1913 Webster]