Dehydration is a loss of too much fluid from your body. The human body needs water to maintain normal functions. If your body loses much more fluid than you are drinking, you become dehydrated. If the loss of fluid is severe, you can become very ill and possibly die.
You lose fluids when you urinate, vomit, have diarrhea, sweat, breathe rapidly, overexert yourself in hot or humid weather, or have a fever.

Along with the fluids, your body also loses electrolytes (mineral salts), which are also important for normal body functions.

The usual causes of dehydration are diarrhea and vomiting. Dehydration can also occur if you do not eat or drink much during an illness or if you do not drink enough during or after strenuous exercise. Medicines that control excess body fluid (diuretics, or "water pills") by causing fluid loss are a common long-term cause. Diabetes can also cause water loss.
Although anyone can become dehydrated, adults who become dehydrated the most easily are older adults, anyone with a fever, people in hot weather, and people with diabetes.