Plain English Meaning
It's the colored part of your eye. It works like a little muscle that opens and closes the black hole in the middle (the pupil) so the right amount of light gets in.
Full Definition
The iris is the colored, ring-shaped muscle at the front of the eye that surrounds the pupil and controls how much light enters by changing the pupil's diameter. In bright light it contracts to make the pupil smaller; in dim light it relaxes to make the pupil larger.
Context Anchor
Seen in night vision and eye anatomy discussions, especially when learning how the eye adjusts between bright light and darkness.
Derivation
From the Greek 'iris,' meaning rainbow, named for the variety of colors the iris can have between people. Knowing this links the word to its visible, colorful appearance in the eye.
Why Pilots Care
Understanding iris action explains why bright lights destroy night vision and why red lighting helps preserve it during .