Pupillary distance is short for PD. It is the distance between the center of your two pupils in millimeters. Your eye care provider will usually measure your PD during an eye exam. 
PD measurement determines where you will look through the lenses and should be as accurate as possible.
The average adult's PD is between 54-74 mm; kids' are between 43-58 mm.
You can get your PD from your eye care provider, or you can measure it yourself.

Eye care providers should list your PD when they fill out a prescription for you. It is the key to have accurate PD measurement, especially if you intend to get progressive lenses. There are several ways to get PD measurement.
1.To ask your eye care provider to double-check the measurement.
2.To measure it yourself with a mirror and millimeter ruler. (Check out the guide) 
3.To have a friend to measure it with a millimeter ruler. (Check out the guide)