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]