Plain English Meaning
Steps a pilot takes to keep their eyes adjusted to the dark so they can see well at night. The eyes need about 30 minutes in low light to reach full night vision, and a sudden bright light resets that process.
Full Definition
The practice of preserving the eye's dark-adapted state before and during night flight by avoiding bright light exposure, using red or low-intensity cockpit lighting, and allowing approximately 30 minutes for the eyes to fully adjust to darkness. Once dark adaptation is lost through exposure to bright light, full recovery again requires the same lengthy adjustment period.