Nominal Pipe Size - Wikipedia
https://en.wikipedia.org/wiki/Nominal_Pipe_Size#NPS_1%E2%81%848_to_NPS_3+1%E2%81%842Nominal 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 2.375-inch (60.3 mm) outside diameter).