Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.[1] “Nominal” refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number (for example – “2-inch nominal steel pipe” consists of many varieties of steel pipe with the only criterion being a 2-inch diameter hole through the center). Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule (Sched. or Sch., for example – “2 inch diameter pipe, Schedule 40”). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). The European and international designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm), in which sizes are measured in millimetres, see ISO 6708.[2] The term NB (nominal bore) is also frequently used interchangeably with NPS.
References:
https://en.wikipedia.org/wiki/Nominal_Pipe_Size
