I would also add that there's often an element of "legal relationship" in the immigrant/expat split. Someone who identifies as an "immigrant" often means, in my experience, that they intend to establish citizenship in the new country -- if not for themself, then certainly for their children. Whereas on the other hand, i think expats often have no intentions of either surrendering their U.S. citizenship OR of establishing citizenship in a new country, but they also certainly have no intention of returning to the U.S. to live: Andy's aunt, for example, who is having all their possessions shipped over to the U.K. now that her husband has finally retired there.

Not that these are fixed definitions by any means! (Despite how much governments might want to make them so -- i'm thinking here of U.S. H-visas (aka, work visas) which state that the holder is maintaining their residency in their home country, a definition that seems explicitly designed just to fuck people over.) Lots of the people i know who live internationally (for lack of a better phrasing) don't fit neatly into either category. I know a few people living in the U.S. who have each been living here for decades, but have never wanted anything other than permanent resident cards (one woman is married to a U.S.ian and has children with dual citizenship, but i don't think she perceives of herself as an immigrant -- she's just sort of living here). And i know lots of people with multiple citizenships, which breaks things up into yet some 4th dimension of discussion.