Prolonged painful involuntary contraction of skeletal muscle. It is sometimes caused by an imbalance of the salts in the body, but is more often a result of fatigue, imperfect posture, or stress. Pain. Possibly inability to perform specific tasks (e.g. of 'occupational cramp' is 'writer's cramp')
Cramps are unpleasant, often painful sensations caused by contraction or overshortening, usually of muscles. Cramps can be caused by cold or overexertion. Illness or poisoning can also cause cramps, particularly in the stomach, which is referred to as colic if it fits particular characteristics.
Cramp sucks, so avoid it by staying hydrated and stretch frequently between sets. If you sweat a lot, you may need to watch your sodium too -- try a specially formulated sports drink for prolonged workouts.