Exit Pupil
The cone of light leaving the scope's eyepiece. Diameter in millimeters equals the objective lens size divided by magnification.
A 5-25×56 scope at 5x has an exit pupil of 11.2mm (56÷5); at 25x it’s 2.24mm. The human eye’s pupil dilates up to about 7mm in darkness, so anything above that is unused light.
For low-light work, larger exit pupil makes the image brighter on the retina. For daylight precision work at high magnification, exit pupil approaches your eye’s daylight pupil size (2-3mm). At that point optical quality matters more than raw exit pupil size.