iris
n. countablen. the colored part of your eye around the black center. It controls how much light enters your eye by changing size.
n. the circular, pigmented membrane behind the cornea of the eye, with the pupil in its center. It regulates the amount of light reaching the retina by adjusting the diameter of the pupil.
Her iris is a striking shade of deep green.
The doctor used a bright light to check how quickly the patient's iris reacted to the change.
In low light conditions, the muscles of the iris relax to dilate the pupil, allowing the maximum amount of visual information to reach the sensory cells of the retina.
From Ancient Greek Ἶρις (Îris, “rainbow”).
Refers to the anatomical structure of the eye; also used in photography to describe the adjustable diaphragm of a lens.