"Family reunification" refers to the act of bringing together family members who have been separated for various reasons, so they can be together again. Full definition
There are INIS guidelines about visa requirements for family reunification for workers (pdf). (citizensinformation.ie)
(B) When a case plan indicates that placement is for purposes of providing family reunification services... (advokids.org)
He said Trump will try to divide the Democrats to force through a draconian anti-immigration reform plan that includes funding for a wall at the Mexican border, an end to a diversity visa lottery that brings in people from around the world and a curtailing of family reunification efforts, which Trump calls «chain migration.» (buffalonews.com)