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]

The Collaborative International Dictionary of English v.0.48:

Senegin \Sen"e*gin\, n. (Med. Chem.)
   A substance extracted from the rootstock of the Polygala
   Senega (Seneca root), and probably identical with polygalic
   acid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Snakeroot \Snake"root`\, n. (Bot.)
   Any one of several plants of different genera and species,
   most of which are (or were formerly) reputed to be
   efficacious as remedies for the bites of serpents; also, the
   roots of any of these.
   [1913 Webster]

   Note: The Virginia snakeroot is Aristolochia Serpentaria;
         black snakeroot is Sanicula, esp. Sanicula
         Marilandica, also Cimicifuga racemosa; Seneca
         snakeroot is Polygala Senega; button snakeroot is
         Liatris, also Eryngium; white snakeroot is
         Eupatorium ageratoides. The name is also applied to
         some others besides these.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Polygala senega
    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]