This is the measure of the visual performance of the binoculars in poor and dim light condition. Twilight factor can be calculated by multiplying the objective lens diameter to the magnification of the lens and then taking the square root of the product. The larger the twilight factor, the more detail you can see in low light. This factor is very much important for the hunters and birdwatchers, since animals are often spotted just before sunrise and after sunset.
Twilight Factor = square root (Magnification Power x Objective Lens Diameter)
Twilight Factor of a 10x50 Binocular = square root (10x50) = 22.5
A twilight factor of 17 or better usually required for reasonable low light