Expat, Short for Expatriate (v): To withdraw (oneself)
from residence in or allegiance to one's native country.
To some the term expatriate and its abbreviation expat, verb and noun, has the connotation of exile; someone who has been thrown out of his native country, has left in protest, or has felt the need to leave because of factors in the native country. An un-choosing of the native country.
In business circles an expat is often defined as a professional whose contract with the company is on the terms of the country he is coming from rather than on the terms of the country where he works. The company holds the employee free of any expenses that may be related to the transfer (differences caused by different tax rules, housing allowances, education allowances for accompanying children, appliances running on different electric systems etc.). In this context a transfer is an employee who is sent to another country but who works under the same type of contract as employees in the that country.
In the community of people who have “withdraw (themselves) from residence in their native countries” the term is applied to any person to whom any of these categories can be applied:
3. Permanent resident
4. Diplomatic transfer
5. Business transfer
6. Asylum seeker
7. Person in exile
8. Accompanying family of any of the above
9. Au pair
11. Exchange student
Expat will normally not be used for soldiers at war, although it does apply to for example US soldiers stationed on US bases in Germany.
The difference between an immigrant and a sojourner is in the intention to stay. People from categories 4,5,9,11 – and accompanying family – can be described as sojourners. People described as immigrants or permanent residents normally don’t plan to return at a time known at arrival - or at all. Refugees, exiles and asylum seekers fall in between. They often hope to return but don’t know when it will be possible.
(I failed to mention Resident Alien. I confess, I am a resident alien. I won't blame you if your associations go towards ET or Aliens vs. Predators but I am a normal Homo Sapiens. Isn't it wonderful with terms that only the immigration authorities would ever use?)
As a general rule, unless you live in a veritable expat community, it is conductive to your integration to behave as if you intent to live there forever. Why would the locals invest themselves in building friendships with you if you have already signaled that you are leaving in a year - or are not sure if you even like the place?