In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Most people can tolerate a 3–4% decrease in total body water without difficulty or adverse health effects. A 5–8% decrease can cause fatigue and dizziness. Loss of over 10% of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs with a 15% and 25% loss of body water. Mild dehydration usually resolves with oral rehydration, but severe cases may need intravenous fluids.

Source: Wikipedia