Fork steerer tubes are referred to by outer diameter - as measured.
Pretty much your only source of confusion is if you would measure at crown seat race, which is a little bit wider than the rest of the steerer.
And for 1" forks the funny business with ISO and JIS crown races.

What sometimes get funky is quill stems. They may refer to both the O.D. of the steerer tube they're supposed to fit, OR the O.D. of the quill.