The Collaborative International Dictionary of English v.0.48:

Cathartic \Ca*thar"tic\, Catharical \Ca*thar"ic*al\, a. [Gr. ?,
   fr. ? to cleanse, fr. ? pure; akin to F. chaste.]
   1. (Med.) Cleansing the bowels; promoting evacuations by
      stool; purgative.
      [1913 Webster]

   2. Of or pertaining to the purgative principle of senna, as
      cathartic acid.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cathartic \Ca*thar"tic\, n. [Gr. ?.] (Med.)
   A medicine that promotes alvine discharges; a purge; a
   purgative of moderate activity.
   [1913 Webster]

   Note: The cathartics are more energetic and certain in action
         that the laxatives, which simply increase the tendency
         to alvine evacuation; and less powerful and irritaint
         that the drastic purges, which cause profuse, repeated,
         and watery evacuations. -- Ca*thar"tic*al*ly, adv. --
         Ca*thar"tic*al*ness, n.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cathartic
    adj 1: emotionally purging [syn: cathartic,
           psychotherapeutic]
    2: emotionally purging (of e.g. art) [syn: cathartic,
       releasing]
    3: strongly laxative [syn: cathartic, evacuant, purgative]
    n 1: a purging medicine; stimulates evacuation of the bowels
         [syn: purgative, cathartic, physic, aperient]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "cathartic":
   abstergent, alleviating, alleviative, analgesic, anesthetic,
   anodyne, aperient, assuasive, balmy, balsamic, benumbing,
   carminative, cleaner, cleaning, cleaning agent, cleaning solvent,
   cleanser, cleansing, cleansing cream, cold cream, cream, deadening,
   demulcent, dentifrice, depurant, depurative, detergent, diuretic,
   dulling, easing, emetic, emollient, enema, expurgatory, holystone,
   laxative, lenitive, lotion, lustral, mitigating, mitigative,
   mouthwash, nauseant, numbing, pain-killing, palliative, physic,
   pumice stone, purgative, purge, purging, purificatory, purifier,
   purifying, relieving, remedial, rinse, shampoo, soap, softening,
   solvent, soothing, subduing, synthetic detergent, tooth powder,
   toothpaste, wash