[jwilliams]

In my experience, people have come to use expat to describe someone who is temporarily or provisionally living elsewhere (e.g. Gone to London to work for a few years). Immigrant is often used for people who have moved with a permanent intention.

[davidp]

Agreed.

An expat is someone "on assignment", who has and expects strong ties with their home country. People who fit this description tend to frequently seek help from their own consulates, for example. It's particularly relevant for Americans because we tax ourselves even when overseas.

An emigrant (as seen from the source country) or immigrant (as seen from the destination country) is someone who has (usually permanently) moved and now expects primary services and sovereignty from their new home country.

I highly doubt that the distinction is strictly an anglophone concept, although perhaps it's related that they sound similar and have similar roots.

[usrusr]

That's the difference, and it is inevitably an international one (e.g. we happily adopted the English term "expat" here in Germany) because it only exists across borders. But the difference gets muddied a lot because one is associated with much higher status than the other and everybody will naturally try for the higher status label if there is the remote chance of getting away with it.

Higher status? "People who hang out with diplomats", try beating that. If you routinely dine at your embassy you'd sure be considered "expat" even if you are from Afghanistan. Of course only a tiny fraction of expats would actually do this, but it illustrates the idea. A more practical line of distinction: would they seems their children to regular local schools? If they insist on private "international schools", then it's "expat", otherwise only if a return date is known.