A pupil is the opening in the centre of the eye that allows light to enter the eye and reach the retina, which is the light-sensitive layer of tissue at the back of the eye. The size of the pupil changes depending on the amount of light entering the eye. When it is dark, the pupil dilates (gets bigger) to allow more light in, and when it is bright, the pupil constricts (gets smaller) to let less light in.
puppil, peupil, popill