The term, in its broadest sense, may cover a multitude of cases. Most commonly, it refers to economic migrants, who typically travel (either legally or illegally) to a country with a stronger economy than the one in which they hold citizenship. Those that are legal may be either full-fledged immigrants or may be in the host country on a conditional work permit.
The term can also include international experts working out-of-country (usually, but not always, legally) and any number of cases in between. For example, in recent years in the USA there has been much controversy over whether H-1B visas, intended to bring highly skilled workers to fill gaps in the domestic labor pool, are instead being used to bring in skilled, but otherwise unexceptional, economic migrants as cheap labor to fill jobs that could readily be filled domestically.
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See also Immigration.