What is your ethnicity?

You're probably actually asking about ancestry and not so much ethnicity, since people are answering based on nationalities. My ethnicity is Hispanic, but my ancestry is a big mix since I'm from the West. My mom did the ancestry test and it was about 74.2% Southern European with Spanish/Portuguese making up about 55% of that so I'd say that's the primary.

An ethnic group or ethnicity is a category of people who identify with each other on the basis of a belief of common descent.[1] Descent-based attributes such as a common language, ancestry, history, society, culture, nation, religion, race or social treatment within their residing area may define the ethnic group.[1][2][3] Ethnicity is sometimes used interchangeably with the term nation, particularly in cases of ethnic nationalism, and is separate from, but related to the concept of races.

Ethnicity is usually an inherited status based on the society in which one lives. Membership of an ethnic group tends to be defined by a shared cultural heritage, ancestry, origin myth, history, homeland, language or dialect, symbolic systems such as religion, mythology and ritual, cuisine, dressing style, art or physical appearance. Ethnic groups often continue to speak related languages and share a similar gene pool. By way of language shift, acculturation, adoption and religious conversion, it is sometimes possible for individuals or restricted groups to leave one ethnic group and become part of another.

Ethnic groups may be subdivided into subgroups or tribes, which over time may become separate ethnic groups themselves due to endogamy or physical isolation from the parent group. Conversely, formerly separate ethnicities can merge to form a pan-ethnicity and may eventually merge into one single ethnicity. Whether through division or amalgamation, the formation of a separate ethnic identity is referred to as ethnogenesis.

I don't think race and ethnicity are synonymous:

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I expect the concept of ethnicity from a cultural viewpoint has also broken down a lot because of the internet and globalisation.

Race and ethnicity aren't the same. I never mentioned race in my post. I mentioned my ethnicity and my ancestry.

No it's just you mentioned nationality and that's usually included as part of it. I guess different people have different definitions.

Oh this is one of those moments where nationalities are grouped into ethnicities. But ethnicities aren't nationalities. So people can be mexican, venezuelan, argentinian etc and that would make their ethnicity hispanic/latino.

While Europe I think has scandinavians, balkanic, the celtics...

Oh I see. I feel that would get confusing with all the European subgroups and then there are multiple within nations as well. Also thinking about it on UK census forms they reduce it to white British and white other.