Eliminating chain migration, the process that allows family members of immigrants to come to the U.S., has been a target of President Trump.

Trump has often lambasted the program and called for its end, including in his first State of the Union address. And at a bipartisan meeting with lawmakers earlier this month, Trump said any immigration reform package would have to include a solution to chain migration in order for him to be on board.

What is it?

“Chain migration” occurs when immigrants enter the U.S. through sponsorship from family members already settled in the country. Relatives are able to apply for visas through this system.

Another term for this system is “family reunification.”

Green card holders are able to petition for their spouse and unmarried children to become permanent U.S. residents, according to U.S. Citizenship and Immigration Services.

U.S. citizens can petition for visas for additional family members.

Critics have argued that the system is flawed in that it could trigger an endless flow of family members emigrating into the country.

What has Trump said about it?

While giving the State of the Union address, Trump walked through four pillars of his immigration reform plan, one of which includes stopping chain migration.

"Under the current broken system, a single immigrant can bring in virtually unlimited numbers of distant relatives," Trump said. "Under our plan, we focus on the immediate family by limiting sponsorships to spouses and minor children."

The president also spoke about two New York City terror attacks which he said “were made possible by the visa lottery and chain migration.”

Accused terrorist Sayfullo Saipov reportedly immigrated via the Diversity Immigrant Visa Program. Saipov is accused of mowing down pedestrians in October on a crowded bike lane in Manhattan, killing eight people and wounding a dozen others.

Terrorist suspect Akayed Ullah reportedly immigrated through chain migration. Ullah allegedly set off a pipe bomb in a corridor of the Times Square subway station in December.

Trump has also often tweeted his call to end chain migration.

“Some people come in, and they bring their whole family with them, who can be truly evil,” Trump said in one online post.

Fox News’ Zoe Szathmary and The Associated Press contributed to this report.