The Collaborative International Dictionary of English v.0.48:

Senecas \Sen"e*cas\, n. pl.; sing. Seneca. (Ethnol.)
   A tribe of Indians who formerly inhabited a part of Western
   New York. This tribe was the most numerous and most warlike
   of the Five Nations.
   [1913 Webster]

   Seneca grass(Bot.), holy grass. See under Holy.

   Seneca eil, petroleum or naphtha.

   Seneca root, or Seneca snakeroot (Bot.), the rootstock of
      an American species of milkworth (Polygala Senega)
      having an aromatic but bitter taste. It is often used
      medicinally as an expectorant and diuretic, and, in large
      doses, as an emetic and cathartic. [Written also Senega
      root, and Seneka root.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

Seneca snakeroot
    n 1: eastern North American plant having a terminal cluster of
         small white flowers and medicinal roots [syn: Seneca
         snakeroot, Seneka snakeroot, senga root, senega
         root, senega snakeroot, Polygala senega]