Source : Webster's Revised Unabridged Dictionary (1913)

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. ({C. acutifolia}, {C. angustifolia}, etc.).
      They constitute a valuable but nauseous cathartic
      medicine.

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

   {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.

Source : WordNet®

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