The term "dystonia" refers to a state of abnormal (too much or too little) muscle tone. There are many forms of dystonia. Dystonia disorders cause involuntary movements and prolonged muscle contraction, resulting in twisting body motions, tremor, and abnormal posture. These movements may involve the entire body, or only an isolated area, as in wordprocessor's cramp.
Similar focal dystonias have also been called writer's cramp, typist's cramp, pianist's cramp, musician's cramp, and golfer's cramp. The mechanism by which they arise is the same as in wordprocessor's cramp -- an oft-repeated repetitious movement.