The Collaborative International Dictionary of English v.0.48:

Practitioner \Prac*ti"tion*er\, n. [From Practician.]
   1. One who is engaged in the actual use or exercise of any
      art or profession, particularly that of law or medicine.
      --Crabbe.
      [1913 Webster]

   2. One who does anything customarily or habitually.
      [1913 Webster]

   3. A sly or artful person. --Whitgift.
      [1913 Webster]

   General practitioner. See under General, 2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

practitioner
    n 1: someone who practices a learned profession [syn:
         practitioner, practician]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "practitioner":
   actor, agent, architect, author, creator, doer, executant,
   executor, executrix, fabricator, maker, medium, mover, operant,
   operative, operator, performer, perpetrator, prime mover, producer,
   subject, worker