The Collaborative International Dictionary of English v.0.48:

Cohosh \Co"hosh\, n. (Bot.)
   A perennial American herb (Caulophyllum thalictroides),
   whose rootstock is used in medicine; -- also called pappoose
   root. The name is sometimes also given to the Cimicifuga
   racemosa, and to two species of Act[ae]a, plants of the
   Crowfoot family.
   [1913 Webster] Cohune