The pupil is the circular aperture in the center of the iris. Light enters the interior of the eye through the pupil. Its size is regulated by the surrounding iris, depending on the light conditions in the environment:

When there is a lot of light, the pupil through the iris becomes smaller and when the environment is dark, it becomes larger. This is an unconscious process by which the incidence of light on the retina can be adjusted. The pupil is also called the "eye hole".

Pupil shape of a living being

Cats, for example, unlike humans, have a vertical slit-shaped pupil instead of a round one. This is due to the fact that the sphincter muscle of the iris is scissor-grid-like instead of circular. The pupil shape of a living being is adapted to the lens properties: For example, only living beings with a round pupil have a multifocal lens (lens with multiple focal points).