Well, generally speaking, Pupillary distance in glasses means the distance between the pupil of your right eye and the pupil of your left eye. And in common ways, it has to be very accurate, or it will just have some trouble on your glasses. A proper measurement of a PD ensures that the lenses are positioned correctly in relation to the pupil centers. Of course, the standard measurement is taken in millimeters. For many people, to get the accurate number, it is necessary to measure it by some professional people. So maybe you can get it in some eye store.
Yes, a lot of people choose to buy eyeglasses online because of the suitable price. However they need to provide the accurate pupillary distance which could provide the suitable eyeglasses for you. Usually the adult female common pd is 56 mm - 62 mm. While the adult male common pd is 58mm to 68mm. You could better go to the eyeglasses store to have a check on the eyes pupillary distance.
The pupillary distance refers to the distance between the centers of the two eyeballs. When the two eyes of human beings are watching the same object, the object will leave images both on the two retinas of the two eyes, and the two images will overlap in the visual center of the brain and form a complete sole image with three dimension, and this function is called binocular single vision. Pupillary distance has to be measured when people want to fill a prescription for their glasses, and there are three kinds of pupillary distance to be measured, that is, distance PD, near PD, and common PD. The distance of the two centers of the glass lenses should match to the PD, or else people will feel uncomfortable even when the prescription is accurate. Different people have different pupillary distances, and the normal PD is usually from 58 to 64mm. Besides, the pupillary distance is usually affected by the shape of face, the position of the eye socket, and if a person has a bigger face and bigger eye sockets, his or her pupillary distance will also be greater, but as long as the vision spot is in the center of the pupil and the eye position is normal, it will not be a problem.