Q: Why is a stop called a stop?
A: Because on older cameras, the adjustments for shutter and aperture would “click”, ie come to a stop, for each halving or doubling of the light. Say from 1/125th second to 1/60th, or from f/8 to f/5.6. Click – click – click. This made it easier to adjust your exposures a known amount, just like a guitar with frets is easier than a violin with “continuous” adjustments.
Oh and bonus question:
Q: Why f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, and so on?
A: First, note that it is not “f 8”, but “f/8”, meaning f (the focal length) divided by the subsequent number. And like the “main” shutter speed numbers, these main aperture numbers are also chosen to give you double, or half, the light with each subsequent number. And to halve the light coming through a circle you need to divide the radius not by 2 (that would give you a quarter of the light) but by the square root of 2, or 1.414. (Since area = Π r ²). So the ratio between these funny numbers is that, 1.4. Meaning each larger number (because we are dividing by this number) gives you half the light entering the lens.