Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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