"Nominal" refers to a non-specific pipe material that uses a non-dimensional number to identify the diameter of the hole. Specific steel pipes are identified by the pipe diameter and another non-dimensional number indicating the wall thickness, called "Sched." or "Sch." NPS is often incorrectly called "National Pipe Size" due to confusion with the American pipe thread standard "national pipe straight" (also abbreviated as "NPS"). The European and international equivalent of NPS is DN (diamètre nominal/nominal diameter/Nennweite), where dimensions are in millimeters.
