A belief, mostly by left-wingers, that there should be free movement of people from one country to another and they shouldn't need documentation for citizenship. This is argued because many liberals believe that it is "too hard" for illegal immigrants to become citizens. Enacting this policy is too risky for a country because it would not be able to monitor what kind of trash enters the country, not to mention the tax burden it would bring.
Jonny: "Fuck Donald Trump and his wall. Why does the U.S. make it so hard to keep immigrants out of the country. We should have open borders."
Hal: "While I do believe that legal immigration is good for any country, it is an idiotic idea to have open borders. How would you know that ISIS or Zika isn't getting in the country?".