F-stop

The f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. It is also known as the focal ratio, f-ratio, or f-stop. It is the reciprocal of the relative aperture. The f-number is commonly indicated using a hooked f with the format f/N, where N is the f-number.

The f-number N or f# is given by:

where ${\displaystyle f}$ is the focal length, and ${\displaystyle D}$ is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the entrance pupil diameter in terms of f and N. For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2, expressed by writing "f/2", and the aperture diameter is equal to ${\displaystyle f/2}$, where ${\displaystyle f}$ is the focal length.

...
Wikipedia