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]