The word alien has several meanings:

1. Something completely unfamiliar or foreign. When you visit a place where nobody speaks your language and the customs seem totally strange, it might feel alien to you. Scientists exploring deep ocean trenches discover creatures so bizarre they look alien. The word captures that sense of something being so different it feels like it comes from another world.
2. A being from another planet. Science fiction stories often feature aliens visiting Earth: creatures from distant solar systems with unusual abilities or appearances. While we haven't discovered real aliens yet, scientists actively search for signs of life beyond Earth. When someone talks about alien life forms or extraterrestrial life, they mean living things that evolved somewhere other than our planet.
3. A person living in a country where they're not a citizen. If someone moves from Mexico to the United States but hasn't become a U.S. citizen yet, immigration law calls them a resident alien. This legal term simply describes their citizenship status, not whether they belong in the community. After living in a country for several years and meeting certain requirements, a resident alien can often apply to become a citizen.

Whether describing strange creatures, unfamiliar experiences, or citizenship status, alien always points to something or someone from outside the familiar circle.