PD or pupillary distance refers to the distance in millimeters between the center of one pupil to the center of the other. Having a correct PD on your prescription insures that you are looking through the ideal spot in your lenses. If this number was not provided on your prescription by your eye care professional, you can measure it yourself.
Note that the average PD is between 57 and 65mm, and an average of 62mm. For bifocal and progressive lenses, two PD values are needed. In these cases, EyeBuyDirect recommends getting your PD directly from your eye care professional.