Objective Lens
The forward lens of a scope, the one facing the target. Its diameter in millimeters governs how much light enters.
A scope written “5-25x56” has a 56mm objective. Larger objectives gather more light, which matters most in dawn/dusk shooting. They also raise the scope higher above the bore, which can force taller rings and complicate cheek weld.
Past about 50mm there are diminishing returns under normal daylight. For precision rifle competition where light is rarely a problem, 50mm or even smaller is fine.