The Collaborative International Dictionary of English v.0.48:

Diuretic \Di`u*ret"ic\, a. [L. diureticus, Gr. ?, fr. ? to make
   water; ? through + ? to make water, fr. ? urine: cf. F.
   diur['e]tique.] (Med.)
   Tending to increase the secretion and discharge of urine. --
   n. A medicine with diuretic properties.
   [1913 Webster]

   Diuretic salt (Med.), potassium acetate; -- so called
      because of its diuretic properties.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diuretic
    n 1: any substance that tends to increase the flow of urine,
         which causes the body to get rid of excess water [syn:
         diuretic drug, diuretic, water pill]