This is a hot topic among expat communities. As someone that was an expat for 12 of the past 20 years, I have some strong feelings about it. Expat implies (and literally means) someone outside of their home country. The key element here is “home.” An immigrant is someone who leaves their “home” for a new home country permanently.
For example, I was an American expat in France for the past 4 years (and in Mexico and Asia before that,) only just recently having returned to the US. I had no intent on permentantly settling in France. There are plenty of long term German, French and Dutch, Japanese expats in China (not anglophone obviously,) but they are almost entirely never intending (or able to,) become citizens of China. They are expats. They work a job for 2+ years, but they never intend to become Chinese. Nor are they floating around city to city each year picking up seasonal jobs as is the case for migrants.
An H1B worker from India in California is also an expat, up until the point where they are intending to become a citizen and stay permenantly — in which they become immigrants. An Indian H1 who is after a green card though, becomes an immigrant because of their intent to permantly change home countries.
“Migrant” worker versus expat is another nuance and it is because historically, migrant workers, by definition, migrate temporarily based on the availability of seasonal work. Migrant workers, by definition don’t stay in one place very long — they would typically work a farming season and then move to the next place where a harvest is coming.. like migratory birds going south for the winter. A migrant worker doesn’t stay in one place very long: they migrate depending on where the work is and ideally, return home between temporary jobs. America used to have a robust guest worker program in the 1950s-1960s and people from Mexico would cross the border to work the harvests, then go back home when the season was finished.
In short: an expat is someone living for some longer length of time in a country outside their own with the intent to eventually go back home. Expat is a “temporary” status characterized by long stints in one place away from home. An immigrant is someone that leaves their origin country permenantly. A migrant is someone that follows the harvest (i.e. seasonal jobs) and returns home regularly.
Some people have attached racial or class distinctions to those terms which is unfortunate because such distinctions aren’t accurate — they are the invention of (mostly white) people that seem to find offense in everything. This topic has lit expat Facebook groups and forums up with white expats attempting to put virtue Signal everyone else while ignoring the actual nuances involved. It’s much easier to engage in pseudo-intellectual outrage in order to assuage their own feeling of expat “privilege.” It was exhausting being around those sorts of people. Some white woman who is the wife of a British business executive on a full expat package in Shanghai trying to convince others that she is an immigrant is ridiculous when she’s talking about retiring one day to her country house in Yorkshire.
The TLDR is that one’s status of expat, migrant or immigrant is one of intent, not class or national origin.