Dilatation
The process of enlargement or expansion.
For example, dilatation of the eye is the process by which the pupil is temporarily enlarged with special (mydriatic) eye drops.
This allows the eye care specialist to better view the inside of the eye.
The word 'dilation' means the same thing as 'dilatation'.
Both come from the Latin 'dilatare' meaning 'to enlarge or expand.'