Focal ratio
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system (be it a camera lens or telescope) is the ratio of the lens's focal length to the diameter of the entrance pupil.
Focal ratio is a dimensionless number that serves as a quantitative measure of lens speed, and a very important concept in photography and even more in astrophotography.