I always just thought that "expat" meant a person dispatched by a company or organization to do work in another country and with an expectation that the worker in question moves back to their country of origin when their contract is over. Whereas an immigrant tends to be a person who moves permanently / with an idea to settle in another country. Though i will agree with the other commenters here that my definition of "expat" is essentially a whites-only word for "migrant worker".