Abdullah Alghazali, right, hugs his 13-year-old son Ali Abdullah Alghazali after the Yemeni boy stepped out of an arrival entrance at John F. Kennedy International Airport in New York on Feb. 5, 2017.
Alexander F. Yuan, AP

As the White House and Congress grapple over immigration legislation, the term "chain migration" has quickly become part of the D.C. lexicon.

President Trump has blasted the practice as a massive immigration loophole that terrorists and "truly evil" people can exploit to infiltrate the U.S. Democrats have defended it as a cornerstone of America's immigration history.

So what exactly is it?

Put simply, "chain migration" is a derogatory term used to describe the ability of U.S. citizens and green card holders to bring their extended family into the country.

A majority of the roughly one million foreigners who are allowed to enter the U.S. to become permanent residents each year are approved because they're related to Americans.

A U.S. citizen from New York City, for example, can sponsor her husband in Nepal for a green card. A green card holder from San Francisco can sponsor his daughter in Senegal. An American in Illinois can sponsor her niece in Israel.

Trump says this process opens the door for too many uneducated, under-qualified, and possibly dangerous, immigrants to legally enter the U.S. He has been hammering that point ever since police arrested a 27-year-old man accused of setting off a pipe-bomb in a commuter tunnel near Times Square in December. That suspect, Akayed Ullah, has been living in Brooklyn since 2011 after entering the country on an F-4 visa, which is granted to brothers, sisters, nieces and nephews of U.S. citizens. 

Here's a list of the different categories of family-based immigration and how many people entered the U.S. in 2017 through each category, according to the State Department:

  • Spouse  — 112,631
  • Minor child  — 51,327
  • Parent  — 85,280
  • Unmarried, adult son or daughter of U.S. citizen, and their children — 22,807
  • Spouse or child of green card holder and their children  — 106,899 
  • Married child of U.S. citizen, their spouse and children  — 20,716
  • Brother or sister of U.S. citizen, their spouse and children — 61,733

Democrats have defended the idea of family migration, arguing that immigrants have come America as family units since the nation's founding. Family members become support networks for immigrants as they start learning English, going to school or entering a foreign labor force.

And they say its unfair to punish the hundreds of thousands of foreigners who enter the U.S. each year and become law-abiding members of society because of the actions of one would-be bomber.

Despite those defenses, it appears Democrats will have to give up some portions of "chain migration" in order to get Republicans to pass a bill to protect DREAMers from deportation. Trump has ended an Obama administration program to protect from deportation immigrants who were brought to the U.S illegally as children, and has given Congress until March to figure out an alternative plan to protect these so called "DACA" immigrants.

Most people agree that Americans should be allowed to bring their immediate relatives (spouse, minor children and parents) along with them. But should their married, adult children get to come along? What about their brother and sister? Or their nieces and nephews?

Those are the questions that negotiators will be hammering out in the days to come.