Eyepiece or ocular diameter is the outside (housing) diameter on the rear side of a rifle scope, directly where you look in the rifle scope itself.
The outside diameter of the eyepiece does not affect the quality of the picture, but it is an important information for users of bolt-action rifles. This dimension is important when mounting the rifle scope because you have to take into account how far your bolt handle is coming up when reloading. If the eyepiece diameter is too big, it can happen that the bolt hits the eyepiece.
The eye-piece diameter is also an important information when buying a wide-angle rifle scope and the suitable mount for it, because the eyepiece is usually bigger than the main tube, so consequently the user has to calculate the height for the mount from the eyepiece diameter and not from the objective diameter as usually.
Source: Revija Lovec