Your eyes are set at a particular distance from each other, which can vary from person to person. The distance between the center of one pupil to the center of the other is referred to as Pupillary Distance. Typically, the Pupillary Distance falls in a range between 54 and 65 mm. As children grow up, their PD keeps on changing but once they become adults, this value remains constant.
This infographic from Marvel Optics looks at how to measure your PD.