The pupil is the circular aperture in the centre of the iris. Light enters the interior of the eye through the pupil. Its size is regulated by the surrounding iris, depending on the ambient light conditions:
In bright light, the pupil becomes smaller due to the iris, and in dark surroundings it becomes larger. This is an involuntary process that allows the amount of light entering the retina to be adjusted. The pupil is also referred to as the “aperture”.
Pupil shape in living organisms
Cats, for example, unlike humans, have a vertical slit-shaped pupil rather than a round one. This is because the sphincter muscle of the iris runs in a scissor-lattice pattern rather than circularly/round. The pupil shape of a living organism is adapted to the properties of the lens: for example, only organisms with a round pupil have a multifocal lens (a lens with multiple focal points).