Alien means belonging to a different country, race, or group, usually one you do not like or are frightened of.
e.g. He said they were opposed to the presence of alien forces in the region.
You use alien to describe something that seems strange and perhaps frightening, because it is not part of your normal experience.
e.g. His work offers an insight into an alien culture.
If something is alien to you or to your normal feelings or behaviour, it is not the way you would normally feel or behave.
e.g. Such an attitude is alien to most businessmen.
An alien is someone who is not a legal citizen of the country in which they live.
e.g. Both women had hired illegal aliens for child care...
e.g. When war broke out, he was interned as an enemy alien.
In science fiction, an alien is a creature from outer space.