The distance (measured in mm) between the center of your pupils of your eyes when looking far away in the distance. If the prescription lenses are not set at the same distance as the distance between your eyes, then an unwanted prism is induced which may cause eyestrain. In lower prescriptions, the amount of prism induced will be of no consequence and will not cause eyestrain. DO NOT measure your own PD. A PD measurement MUST BE measured by an authorized optical professional only.