Dehydration: Excessive loss of body water. Diseases of the gastrointestinal tract that cause vomiting or diarrhea may, for example, lead to dehydration. There are a number of other causes of dehydration including heat exposure, prolonged vigorous exercise (for example, in a marathon), kidney disease, and medications (diuretics).
One clue to dehydration is a rapid drop in weight. A loss of over 10% (15 pounds in a person weighing 150 pounds) is considered severe.
Symptoms and signs of dehydration include increasing thirst, dry mouth, weakness or lightheadedness (particularly if worse on standing), and a darkening of the urine or a decrease in urination. Severe dehydration can lead to changes in the body's chemistry, kidney failure, and become life-threatening.
SOURCE: MedTerms.com. Dehydration.