An expatriate is someone temporarily or permanently in a country and culture other than that of their upbringing and/or legal residence. Expatriate culture, such as that of the British in India or East Africa can become quite distinct and is often subject to parody and ridicule. Expatriates often find, on returning to their country of origin, that it has developed in ways they find incomprehensible; leading to a sense of alienation and anomie.
The difference between an expatriate and an immigrant is somewhat blurry; in some ways it is a state of mind, with immigrants committing to becoming a part of their new country, whereas the expatriate sees themselves, and is perceived, as living in a foriegn land.
The term is used more often in the context of Westerners living in non-Western countries.
