Expatriate vs Immigrant

Researching the foreign graves at Aoyama Cemetery, I notice that I refer to the people buried there as “expatriates” but not as “Japan immigrants.” Yet many of them lived here for decades before expiring; some had honors heaped upon them by the Japanese government; some married into Japanese families. They were settled permanently; they were iimmigrants.
The dictionary defines immigrant as “a person who leaves one country to settle permanently in another.” It’s too clinical a definition; I feel that there is something more to immigration.
Take me as a case in point. I’m an American expatriate and although I have no intention of returning to the US or of leaving Japan, I don’t consider myself a Japanese immigrant. I don’t know that I’ll ever be a Japanese immigrant, no matter how long I’m here.
I have a friend in Pennsylvania who is British but has lived in the US for about 25 years. She is permanently settled, married to an American, owns property, has a green card but is not naturalised into US citizenship. I don’t think of her as an American immigrant; she’s a British expatriate.
What is it that turns an expatriate into an immigrant? Perhaps it is letting loose the final tie to your homeland. Making an irrevokable and official renunciation of the old stomping ground. Adopting the culture, language and lifestyle of your adopted nation. Or perhaps all it takes is an authorised acceptance or permanent recognition from the government.
Truly, I do not know. What do you think?