Victoria was the capital city of the North American Union. It was named after Queen-Empress Victoria of Great Britain, and located at the northeastern tip of Virginia Province, across the Potomac River from Georgestown, Maryland.
While Victoria functions in the storyline as an analog of Washington, DC, it is referred to several times as being on the south side of the Potomac River, rather than the north side as OTL Washington is, suggesting that it comprises the Virginia regions of Arlington County and perhaps nearby Alexandria as well.
The NAU had already existed for several decades by the time Queen-Empress Victoria, its capital's namesake, was born. Either the NAU had a different capital before then, or the capital city's name was changed to Victoria from something else; the novel does not address this point.