Dehydration, in medical terms, refers to a physical condition in which the body's water content and electrolytes (essential body salts) drop to dangerously low levels. When this occurs, the blood becomes more concentrated while its volume decreases. Dehydration can often be fatal when the body loses 15–20% of its water.

Symptoms of dehydration include extreme thirst, dry lips and tongue, dry and wrinkled skin, increased heart rate, elevated body temperature, and dizziness. Persistent vomiting, diarrhea, and excessive sweating are common causes. Overexposure to the sun or excessive heat can also lead to dehydration. The use of diuretics and other medications that remove excess water and electrolytes from the body may, in rare cases, also cause dehydration. In treating dehydration, physicians aim to replenish lost fluids and electrolytes by having the patient drink mineral water and other electrolyte-rich liquids. In severe cases, these fluids may be administered intravenously.