Objective Diameter
The width of a scope's front lens in millimeters, which governs light gathering, the exit pupil, and how high the scope must be mounted.
Objective diameter is the measured width of the front objective lens, printed as the second number in a scope spec such as 5-25x56, where 56 is the diameter in millimeters. A larger front element gathers more light and yields a brighter image in low light, which is why long-range and dawn-and-dusk optics tend toward fifty and fifty-six millimeter glass. The tradeoff is added weight, a bulkier objective bell, and a higher mount.
The diameter also sets the exit pupil, which is the objective diameter divided by the magnification, so a wider lens keeps a usable exit pupil at higher power. Because a big bell needs clearance above the barrel, objective diameter directly drives the ring height a build requires, and taller rings raise the sight line above the bore. Bigger is not automatically better, since glass quality and coatings often matter more than raw millimeters for the image a shooter actually sees.