The ratio of focal length to clear aperture, generally written as the letter "f", followed by a slash, followed by the ratio, like this: f/8. Such an "f/8" system (pronounced "eff eight") has a focal length equal to eight times its clear aperture.
Focal ratios which are large numbers are said to be long or slow; those which are small numbers are said to be short or fast. There are no sharp lines for determining when to use these terms, but among contemporary astronomical telescopes, any telescope with a focal ratio slower (bigger number) than f/12 would probably be said to be slow, and any telescope with a focal ratio faster (smaller number) than f/6, would be said to be fast.
Fast systems often have more aberrations away from the center of the field of view, than do slow ones. Fast systems are generally more demanding of eyepiece designs, than are slow ones: In practical terms, fast systems generally require more expensive eyepiece designs than do slow ones.

This definition is in context to Telescope. See more contextual defintions for Focal Ratio.