the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture
See also T-number
f-number in American English
(ˈɛfˌnʌmbər)
noun
Photography
a number that represents the relative aperture of a lens, equal to the focal length divided by the effective diameter of the lens aperture: a higher number indicates a smaller opening