Steel pipe (IPS) inside diameters varies all over the place depending on wall thickness while the outside diameter remains constant. Pipe sizes are weird and confusing. As a rule of thumb, hose uses ID, tubing uses OD, and pipe, while the OD is constant, has 'nominal' measurements. A 1" IPS (NPS) pipe has approx 1-3/8" OD. Copper tube/pipe (type L, M, K, DWV) used in plumbing doesn't follow rules for standard tubing in that the size called-out isn't the actual OD.
Often, the term IPS (iron pipe size) and NPS (nominal pipe size) are used to describe the same thing. There are a couple schemes used to call out the wall thickness. Primarly a "schedule" number is used to specify wall thickness. The thinnest being sch 5, and I know it goes higher than as sch 80 (that is the thickess I have used). Steel workers may still use STD, XS, XSS to specify wall thickness (not to be confused with cast iron which I know very little about)
CTS (copper tube size) is used for copper plumbing pipes. It is also used to designate CPVC (I don't know much about cpvc because it isn't allowed in my area).
Tubing size is usually specified by OD. When ordering tubing, the actual OD and the wall thickness is given.
Hose size is specified by ID.
There is also an SDR (standard dimension ratio) scheme used for pipe.I only used SDR pipe once and I really don't know much about it; it was specified by an engineer. Technically the cpvc in CTS uses an SDR scheme. PEX also is sized by a SDR scheme.