Definition
The pupil is the adjustable opening at the center of the iris of the eye through which light passes to reach the retina. Its diameter is controlled by the iris, expanding in low light to admit more light and contracting in bright light to limit the amount entering the eye.
Plain English
The pupil is the small black-looking circle in the middle of the eye. It opens wider when it's dark so more light can get in, and shrinks when it's bright so the eye isn't overwhelmed.
Context Anchor
Seen in night vision discussions because pupil size affects how much light enters the eye in darkness.
Derivation
From the Latin pupilla, meaning 'little doll.' The name comes from the tiny reflected image of yourself you can see in another person's pupil when you look closely. Knowing this helps fix the idea that the pupil is the opening you look into, not the colored part around it.
Why Pilots Care
Proper pupil dilation is essential for night vision; sudden bright light causes constriction and temporary loss of dark adaptation.
Grounding Statement
When you walk from bright daylight into a dark cockpit, your pupils widen to let in more light.
Intuition Check
Pupil does not mean a student here. In this context, it means the dark opening in the center of the eye.
Example Sentence 1
After stepping out of the brightly lit hangar, the pilot waited several minutes for his pupils to widen before starting the night preflight.
Example Sentence 2
A bright flashlight beam caused the pupils to constrict rapidly, forcing the pilot to pause until night vision recovered.