The Collaborative International Dictionary of English v.0.48:

mouthwash \mouthwash\ n.
   a medicated solution used for gargling and rinsing the mouth.

   Syn: gargle.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mouthwash
    n 1: a medicated solution used for gargling and rinsing the
         mouth [syn: gargle, mouthwash]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "mouthwash":
   abstergent, cathartic, cleaner, cleaning agent, cleaning solvent,
   cleanser, cleansing cream, cold cream, cream, dentifrice, depurant,
   detergent, diuretic, emetic, enema, gargle, holystone, lotion,
   nauseant, pumice stone, purgative, purge, purifier, rinse, shampoo,
   soap, solvent, synthetic detergent, tooth powder, toothpaste,
   wash