The f in f-stop is the focal length of the lens. The focal length of the lens divided by the diameter of the aperture is the f-stop. It is also called an f-number. When it is written f/8, this means the aperture diameter is 1/8th the focal length. f/8 on a 45mm lens means that the diameter of the aperture is 45/8, or 5.6mm.

These ratios allow us to use the same number series for all lenses. f/8 on a 45mm lens lets in exactly the same amount of light as f/8 on a 100mm lens. However it is important to note that the diameter of the aperture of these two lenses at these settings is very different, therefore aperture and f-stop are not the same thing.

The amount of light let through the aperture
does not depend on the diameter but the area of the aperture.

Typical f-stop scale:
2   2.8   4   5.6   8   11   16   22
Although these numbers may seem irregular, each of the numbers in this sequence (each stop) represents a halving to the right, or doubling to the left, of the aperture area and therefore the amount of light from its immediate neighbour.
NB. The smaller the number the bigger the aperture and vice versa.