The Collaborative International Dictionary of English v.0.48:

Rhubarb \Rhu"barb\, n. [F. rhubarbe, OF. rubarbe, rheubarbe,
   reubarbare, reobarbe, LL. rheubarbarum for rheum barbarum,
   Gr. ??? (and ??) rhubarb, from the river Rha (the Volga) on
   whose banks it grew. Originally, therefore, it was the
   barbarian plant from the Rha. Cf. Barbarous,
   Rhaponticine.]
   1. (Bot.) The name of several large perennial herbs of the
      genus Rheum and order Polygonaceae.
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   2. The large and fleshy leafstalks of Rheum Rhaponticum and
      other species of the same genus. They are pleasantly acid,
      and are used in cookery. Called also pieplant.
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   3. (Med.) The root of several species of Rheum, used much
      as a cathartic medicine.
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   Monk's rhubarb. (Bot.) See under Monk.

   Turkey rhubarb (Med.), the roots of Rheum Emodi.
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WordNet (r) 3.0 (2006):

Rheum emodi
    n 1: Asian herb (Himalayas) [syn: Himalayan rhubarb, Indian
         rhubarb, red-veined pie plant, Rheum australe, Rheum
         emodi]