Plain English Meaning
The black circle in the center of your eye gets bigger so more light can get in. This happens automatically when it gets dark.
Full Definition
The widening of the pupil of the eye, allowing more light to reach the retina. In low-light or dark conditions, the pupil dilates to increase light intake and improve vision; in bright conditions, it constricts to reduce it.
Context Anchor
Seen in night flying discussions when explaining how the eye adjusts after moving from bright light to darkness.
Derivation
From the Latin 'dilatare', meaning 'to make wider' or 'to spread out'. In aviation night-vision discussions, it specifically refers to the pupil widening as the eye adjusts to darkness.
Why Pilots Care
Dilation lets pilots see better in darkness; bright lights can stop or reverse it and reduce .