It’s also called focal ratio.
OK, the Trick-or-Treaters are gone...
The F in f-stop stands for fenestra, the Latin word for window.
Assigned in the late 1800’s with the development of adjustable diaphragm lenses, it was a designation meant to represent the amount of light required to make a perfect exposure on a film with an arbitrary speed, or sensitivity.
If you had a lens rated at f-1, the light from one window would be sufficient to make a perfect exposure.
The light from 16 windows would mean one would have to ‘stop’ down to the f-16 setting on the lens, reducing the amount of light through the lens, in order to make the correct exposure.
The confusion comes from the use of the word ‘focal’, which pertains to focus. An f-50 lens, has a focal distance of 50mm - the image is in ‘focus’, when the rear lens nodal of the lens is 50mm from the film plane. The closer the rear lens nodal (the high point of the rear lens element) is to the film plane, the wider the field-of-view - a 28mm lens is considered ‘wide-angle’, e.g.