The Collaborative International Dictionary of English v.0.48:

Senega \Sen"e*ga\, n. (Med.)
   Seneca root.
   [1913 Webster]

WordNet (r) 3.0 (2006):

senega
    n 1: dried root of two plants of the genus Polygala containing
         an irritating saponin
    2: perennial bushy herb of central and southern United States
       having white flowers with green centers and often purple
       crest; similar to Seneca snakeroot [syn: senega, Polygala
       alba]