Dilation means becoming wider, or opening up. The eye’s pupil dilates when it opens wider to let in more light. This happens in darker or dimly lit conditions.
She dialled late (dilate) in the evening to see if the takeaway was still open.
The pupil on the right is very wide – this is often referred to as dilation of the pupil. On the left, the pupil has reduced in size, which is known as constriction of the pupil.