<p><strong>Noun</strong></p><ul><li>a creature from outer space; extraterrestrial. Often Disparaging and Offensive.</li><li>a resident of one country who was born in or owes allegiance to another country and has not acquired citizenship by naturalization in the country of residence (citizen).</li><li>resident alien, illegal alien.</li><li>a foreigner.</li><li>a person who has been estranged or excluded.</li><li>a plant or animal species not originating where it is found but introduced, sometimes accidentally, from its native environment elsewhere: Imported plants, seeds, or bulbs may harbor unwanted aliens such as foreign insects or the seeds of other plants.</li></ul><p><strong>Adjective</strong></p><ul><li>unlike one's own; strange; not belonging to one: In English class, our teacher treated all slang as alien speech.</li><li>residing under a government or in a country other than that of one's birth without having or obtaining the status of citizenship there.</li><li>belonging or relating to noncitizens: Under certain conditions, seizure of alien property is authorized.</li><li>opposed; adverse; hostile (usually followed by to or from): Their manifesto is full of ideas alien to modern thinking.</li><li>extraterrestrial (def 1).</li><li>(of a plant or animal species) not native; introduced from a different environment: The bullfrog is native to some Canadian provinces, but is an alien species in British Columbia.</li></ul>