NCI Dictionary of Cancer Terms

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The NCI Dictionary of Cancer Terms features 7,811 terms related to cancer and medicine.

Browse the dictionary by selecting a letter of the alphabet or by entering a cancer-related word or phrase in the search box.

pupil
(PYOO-pul)
The round opening in the center of the iris (the colored tissue that makes the "eye color" at the front of the eye). The pupil changes size to let light into the eye. It gets smaller in bright light and larger as the amount of light decreases.

EnlargeEye anatomy; two-panel drawing shows the outside and inside of the eye. The top panel shows outside of the eye including the eyelid, pupil, sclera, and iris; the bottom panel shows inside of the eye including the cornea, lens, ciliary body, retina, choroid, optic nerve, and vitreous humor.

Anatomy of the eye, showing the outside and inside of the eye including the eyelid, pupil, sclera, iris, cornea, lens, ciliary body, retina, choroid, vitreous humor, and optic nerve.