illegal alien vs immigrant

illegal alien

noun
  • A person who is within the boundaries of a political state without the authorization of the government of that state; a national of another country who has entered or stayed without permission. 

immigrant

noun
  • A non-native person who comes to a country from another country to permanently settle there. 

  • A plant or animal that establishes itself in an area where it previously did not exist. 

adj
  • Of or relating to immigrants or the act of immigrating. 

How often have the words illegal alien and immigrant occurred in a corpus of books? (source: Google Ngram Viewer )