The Collaborative International Dictionary of English v.0.48:

Senna \Sen"na\, n. [Cf. It. & Sp. sena, Pg. sene, F. s['e]n['e];
   all fr. Ar. san[=a].]
   1. (Med.) The leaves of several leguminous plants of the
      genus Cassia. (Cassia acutifolia, Cassia
      angustifolia, etc.). They constitute a valuable but
      nauseous cathartic medicine.
      [1913 Webster]

   2. (Bot.) The plants themselves, native to the East, but now
      cultivated largely in the south of Europe and in the West
      Indies.
      [1913 Webster]

   Bladder senna. (Bot.) See under Bladder.

   Wild senna (Bot.), the Cassia Marilandica, growing in the
      United States, the leaves of which are used medicinally,
      like those of the officinal senna.
      [1913 Webster]

WordNet (r) 3.0 (2006):

senna
    n 1: any of various plants of the genus Senna having pinnately
         compound leaves and showy usually yellow flowers; many are
         used medicinally