That would be 2.54 times 1/100th of the distance light travels in a vacuum during 1/299,792,458 of the time it takes for 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium-133 atom, if you prefer.
While there is a 1 inch measure in common use that is as you described, the subject here is EMT. There is no dimension in EMT that is 1 inch by the system you describe. The diameter is close to 1 inch, but it is noticeably different to the naked eye, and for all useful purposes different enough that anything actually 1 inch in diameter is not compatible.
You see a lot of this in the bicycle industry. There are a lot of older standards in use like 9/16” pedal threads, 1 1/8” steerer tubes or 1” (25.4mm) handlebars but any new standard is metric - so bottom brackets, wheels, newer seat post diameters are all metric. It can make for some very strange looking spec sheets.
So yeah, a weird measurement unit. Technically the symbol to be used is a prime symbol (https://en.m.wikipedia.org/wiki/Prime_(symbol)), but what’s used in practice is anything that looks close enough.
