GAY-genn-halt-ten (from the German gegen, against or counter, and halten, stop
Gegenhalten, sometimes called paratonia, refers to an involuntary resistance to passive movement (that is, having someone else move your body parts). Gegenhalten is a sign of numerous conditions, including catatonia, diffuse encephalopathy, or widespread frontal-lobe disorder--it's one of those conditions that can result from almost any neurological disorder. The term is sometimes used to refer to symptoms of Parkinson's disease; examiners will have trouble moving the patient's arms or legs (and if they do move, they'll often show cogwheel rigidity). Other people suggest that Parkinsonian rigidity is not true gegenhalten.
It's worth noting that gegenhalten may well be a misapplication of a normal response. If someone hands you something heavy--say, a tray of food--you'll (unconsciously) adjust your muscles so that your arm doesn't immediately go crashing to the floor. If someone adds or removes additional weight to the tray, you'll continue to adjust so that you don't drop the tray or send it flying into the air. Gegenhalten, in a way, occurs when the patient cannot help but make the same sort of response to a passive movement.