The Collaborative International Dictionary of English v.0.48:

Rhein \Rhe"in\, n. (Chem.)
   Chrysophanic acid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Chrysophanic \Chrys`o*phan"ic\, a.
   Pertaining to, or derived from, or resembling, chrysophane.
   [1913 Webster]

   Chrysophanic acid (Chem.), a yellow crystalline substance
      extracted from rhubarb, yellow dock, sienna, chrysarobin,
      etc., and shown to be a derivative of an anthracene. It is
      used in the treatment of skin diseases; -- called also
      rhein, rheic acid, rhubarbarin, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Rhein
    n 1: a major European river carrying more traffic than any other
         river in the world; flows into the North Sea [syn: Rhine,
         Rhine River, Rhein]