In its broadest sense, an expatriate is any person living in a different country from where he or she is a citizen. In common usage, the term is often used in the context of professionals sent abroad by their companies, as opposed to locally hired staff (who can also be foreigners). The differentiation found in common usage usually comes down to socio-economic factors, so skilled professionals working in another country are described as expatriates, whereas a manual labourer who has moved to another country to earn more money might be labelled an 'immigrant'.
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