"The unauthorized resident immigrant population is defined as all foreign-born non-citizens who are not legal residents. Most unauthorized residents either entered the United States without inspection or were admitted temporarily and stayed past the date they were required to leave. . ."
"Nearly half of the 12 million-plus illegal aliens in America arrived legally with temporary, non-immigrant visas. The Department of Homeland Security (DHS) estimates that a 'substantial' percentage of America’s illegal population is made up of visa overstays — their estimates range from 27 to 57 percent. . ."