What does a lowercase f (f) mean in Optics? (physics)

I know an uppercase F means focal point, but what does a lowercase f mean? Please help!!!!

*This is a physics question*

The 'f' number of a lens is the focal length divided by the diameter of the aperture the light has to pass through. In photography, a high f number is called for when the subject is brightly lighted. This also causes a greater 'depth of field', or range over which the subject will be in focus.