Dilation, also known as pupil dilation is the widening of the pupil of the eye. It is a normal response of the eye in a poorly illuminated environment. The widening of the pupil allows more light to enter the eye so that vision is improved under dim light.
The pupil can also be dilated using medications. This is usually done during eye examinations that require a broad and detailed view of the retina and other structures at the back of the eye.