Um. Check the dictionary. It means to live outside one's own country.

The dctionary defines expat as:

a person who lives outside their native country.

denoting or relating to a person living outside their native country.

someone who does not live in their own country:

Check any dictionary you like. That is the meaning of the word.

When one becomes a citizen of another country, the term is to 'naturalize.'

When one immigrates to another country, i.e. legal permission to stay there, one need to take up citizenship. One becomes a permanent resident. After 5 years, one can generally 'naturalize,' meaning one can take up the citizenship of another country.

One also does not need to renounce the country of one's birth, as some countries allow dual citizenship. I have dual citizenship, immigrated to one (USA), and have relocated to multiple countires. I am an expat.