Nominal diameter is the approximate inner diameter of the pipe. It is a rounded figure easier to use and remember. By prescribing the nominal size of the pipe, all different fittings can be selected based on the same nominal diameter, without physically checking the dimensions and compatibility of each component.
Nominal diameter is not the internal diameter but is similar to it. With reference to the above example the nominal bore of the pipe is 250 mm, but the ID varies from 266.2 mm to 222.3 mm depending on schedule of the pipe.