What does the 'f' letter stand for in photography?, e.g f5, f2.2 etc

[snotneck]

I know the figures stand for the size of the aperture, but what's the letter for?

[Prescott]

Focal length

[kempie]

It is actually a focal stop i.e. the ratio between focal length and apparent aperture.

A Tedious Explanation of the f/stop