Anisocoria is the medical term describing a difference in the size of the pupils of the eyes. In a normal patient, they should be of identical size.
However, in most cases, anisocoria is benign and is not the symptom of any pathology. About 20% of people present with anisocoria at all times and it is almost always a difference of less than 1mm. However, presentation in a patient with a history of the finding, or a larger difference, can be a symptom of a serious medical condition such as Horner's syndrome. Most cases with an underlying cause are due to trauma, oculomotor nerve palsy, or the use of any of a number of pharmaceuticals or illegal drugs.