Definition of Pupil

Pupil: The opening of the iris. The pupil may appear to open (dilate) and close (constrict) but it is really the iris that is the prime mover; the pupil is merely the absence of iris. The pupil determines how much light is let into the eye. Both pupils are usually of equal size. If they are not, that is termed anisocoria (from "a-", not + "iso", equal + "kore", pupil = not equal pupils).

Aside from the pupil and iris, the eye has a number of other components. These components include the cornea, lens, retina, macula, optic nerve and vitreous.