0 a creature from a planet other than Earth -- alieno, -a
1 a person who lives in a country but is not a citizen -- cittadino straniero, cittadina straniera
a resident alien un cittadino straniero
3 relating to creatures from another planet -- alieno, extraterrestre
an alien spacecraft una nave spaziale extraterrestre
4 strange and very different from what you are used to; foreign -- straniero
alien customs.
They found themselves in an alien environment
5 relating to creatures from another planet -- alieno
alien beings.