Aliens are foreigners to their surroundings. The word is commonly used in law to denote non-citizens of the country of their whereabouts, and in other cases such as science fiction, conspiracy theory and new-age discourse to denote extraterrestrial beings. The term can be used in a broader manner to refer to any object or concept. As an adjective it can be used to describe something foreign or something so different as to be incomprehensible or incompatible.

The word alien came from the Latin possessive adjective alienus = "belonging to someone else".

It is used in several fields and may refer or relate to:

• non-native species
• Extraterrestrial life, in both scientific and popular contexts
• a person who is neither a native nor a citizen of his country of residence
• a computing coversion program
• Alien, the 1979 movie
• the alien featured in the above movie and its sequels