Non-United States citizen who is either a non-resident or Permanent Resident Immigrant.
People who owe no allegiance to the British crown i.e anybody who is not a British citizen, a commonwealth citizen, a British subject, a British Dependent Territories citizen, a British Overseas citizen or Irish citizen.
This has a particular meaning in UK immigration and nationality law. Currently, aliens are people who are not British Citizens, Commonwealth citizens, British Protected Persons or citizens of the Republic of Ireland. At different times in history, different groups of people have been categorised as aliens.