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]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "diuretic":
   abstergent, aperient, carminative, cathartic, cleaner, cleaning,
   cleaning agent, cleaning solvent, cleanser, cleansing,
   cleansing cream, cold cream, cream, dentifrice, depurant,
   depurative, detergent, emetic, enema, expurgatory, holystone,
   laxative, lotion, lustral, mouthwash, nauseant, physic,
   pumice stone, purgative, purge, purging, purificatory, purifier,
   purifying, rinse, shampoo, soap, solvent, synthetic detergent,
   tooth powder, toothpaste, wash