mid-14c., "strange, foreign," from Old French alien "alien, strange, foreign; an alien, stranger, foreigner," from Latin alienus "of or belonging to another, foreign, alien, strange," also, as a noun, "a stranger, foreigner," adjectival form of alius "(an)other" (see alias, adv.). Meaning "not of the Earth" first recorded 1920. An alien priory (c.1500) is one owing obedience to a mother abbey in a foreign country.

"foreigner, citizen of a foreign land," from alien, adj.. In the science fiction sense, from 1953.