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.
The city was built in the second half of the 18th century, for the express purpose of being the Union's capital. It was popularly supposed that Victoria would wither if the government ever left it, but by 1995 that did not seem likely to happen at any near future time.
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.
One paradox that the novel does not address is that the city was already a functioning capital before its namesake Queen-Empress Victoria was born. This implies that Victoria was called something else for the first 40 or more years of its existence.