f-number – (also called f-stop) a number that is used to designate the relative size of the aperture in the lens. The usual sequence of f-numbers is: f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f45, f64, etc.
From Photography by Barbara Upton and John Upton © 1981 Second Edition page 379:
f-number A number that equals the focal length of the lens divided by the diameter of the aperture at a given stop. Theoretically, all lenses at the same f-number produce images of equal brightness.
In practical terms, as each f-number increases it allows half as much light to pass through the lens as the f-number before it. As the f-number decreases it allows twice as much light to pass through the lens as the previous f-number.