Plain English Meaning
Your eyes are chemically tuned to whatever light level you're in. Switch from bright to dark (or dark to bright) and you can't see well for a while because your eyes need time to retune.
Full Definition
The state in which the eye's light-sensitive chemicals (photopigments in the rods and cones of the retina) are balanced for the current light level, allowing the eye to see effectively. When light conditions change suddenly, this equilibrium is disrupted and vision is temporarily degraded until the chemicals readjust. Full adaptation to darkness can take around 30 minutes.
Context Anchor
Seen in night vision and scanning discussions, especially when explaining dark adaptation and the effect of bright lights on a pilot’s ability to see outside.