Dilation, pupil (test): Opening the pupil wide enables your eye care professional to see more of your retina, the light-sensitive layer of tissue at the back of your eye.
Dilating (widening) the pupil permits the retina to be examined for signs of disease. To do this, drops are placed into the eye to dilate (widen) the pupil. After the examination, your vision may (or may not) remain blurred and you may (or may not) be bothered by the brightness of the sun for several hours.
The pupil is the opening in the center of the iris, it is not a structure and does not move itself. It is the iris that dilates or constricts and enlarges or restricts the size of the pupil. The size of the pupil determines how much light is let into the eye.
Dilation is the process of enlargement or expansion. Things don't just open in medicine; they dilate. The word "dilatation" means the same thing as dilation. Both come from the Latin "dilatare" meaning "to enlarge or expand."