Anisocoria: Both pupils are usually of equal size. If they are not, that is termed anisocoria (from "a-", not + "iso", equal + "kore", pupil = not equal pupils).

The pupil may appear to open (dilate) and close (constrict) but it is really the iris that is the prime mover; the pupil is merely the absence of iris. The size of the pupil determines how much light is let into the eye. With anisocoria, the larger pupil lets more light enter the eye.