From the CRS (Congressional Research Service) Insight: "In the first seven months of FY2016 [Fiscal Year], apprehensions of unaccompanied alien children (UAC) at the U.S.-Mexico border have approached levels close to what they were in FY2014 when such migration flows reached what were characterized as 'crisis' proportions. Unaccompanied alien children are statutorily defined as children who lack lawful immigration status in the United States, are under age 18, and lack a parent or legal guardian in the United States or a parent or legal guardian in the United States who is available to provide care and physical custody. They most often arrive at U.S. ports of entry or are apprehended along the U.S.-Mexico border. Less frequently, they are apprehended in the U.S. interior and determined to be juveniles and unaccompanied. UAC treatment and processing in the United States are governed by several statutes and a legal settlement."
CRS Insight, IN10493
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html