The 'X' sizing of tippet was originally used for the cutting of catgut in to strips of uniform thickness.
The same sizing convention has since been adopted by many tippet manufacturers for nylon and fluorocarbon. The original sizing was done in fractions of an inch but has also been converted into metric measurements (**the metric diameters can vary slightly between manufacturers but as we're talking about small percentages of a millimeter the variation isn't big enough to lose sleep over).
Below is a simple sizing chart for the most popular sizes of fly fishing tippet. I've kept this basic and not confused it by including breaking strains or poundage as these vary significantly between brands and materials.