The Collaborative International Dictionary of English v.0.48:

Eupatorium \Eu`pa*to"ri*um\, n. [NL., fr. Eupator, king of
   Pontus, said to have used it as a medicine.] (Bot.)
   A genus of perennial, composite herbs including hemp
   agrimony, boneset, throughwort, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Eupatorium
    n 1: large genus of chiefly tropical herbs having heads of white
         or purplish flowers [syn: Eupatorium, genus Eupatorium]