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