Plain English Meaning

Your body doesn't have enough water in it. When that happens, you start to feel tired, foggy, and slow — which is a serious problem when you're flying or instructing.

Full Definition

The critical loss of water from the body, occurring when fluid output exceeds fluid intake. In the flight training environment, dehydration can develop quickly due to high cockpit temperatures, low humidity at altitude, increased breathing rates, and stress, leading to fatigue, headache, dizziness, slowed reaction time, and impaired judgment.