Twilight Factor
A figure derived from objective diameter and magnification that estimates how well a scope resolves fine detail in low light.
The twilight factor is a single number meant to estimate low-light resolving ability, calculated as the square root of the objective diameter in millimeters multiplied by the magnification. A larger objective lens and a higher magnification both raise the figure, which reflects the trade-off between gathering more light and enlarging the detail you are trying to see. A bigger twilight factor suggests you can distinguish finer features as the light fades.
The number is only a rough guide, because it leaves out the quality of the glass and the light transmission that coatings provide, so two scopes with the same factor can perform differently. It also says nothing on its own about the exit pupil, which must stay large enough for your eye to actually use the brightness the optic delivers.