To a Scot, unless your husband's name is Scott, in which case I apologise for making assumptions.

"I'm married to a Briton". That sounds weird.

(https://www.cia.gov/library/publications/the-world-factbook/...)

Briton is widely used in Britain, e.g. in news articles: "Tokyo destroyed by giant reptile, 3 Britons feared dead"

Great parody about the media coverage of 3-11 there.

Disagree.

"I"m married to a Brit"

Too bad you can't do the same trick with Japanese; it's become politically incorrect (even racist) to use the word "Jap".

What's amusing is that in Japan many Japanese are totally unaware of the political incorrectness of it. In Japan I sometimes hear people use 'Jap' as slang, in store or product names, or just in normal language by extension of how other countries are abbreviated in english. In a country that is mostly japanese, it just loses its racist power.

Is there a proper word to describe this? For example:

* "Jim is British" - "British" is the demonym

* "Jim is a Brit" - "Brit" is the ____

I'm not sure what the correct google search term would be to find out this.

My $partner is Japanese/British/American ...

"My wife is from Britain", "My husband is British" ? Maybe these are now archaic

"From" no longer implies ethnicity and may even not imply culture.