Although there is no formal definition, a sanctuary city or state typically refers to local and state governments imposing limitations that restrict their cooperation with the federal government when it comes to immigration enforcement.

States, counties and cities across the U.S. have adopted policies to limit cooperation with immigration agents who seek to deport undocumented immigrants.

The Department of Justice is now threatening to prosecute officials who protect the status of "sanctuary cities", a move the ACLU calls "unconstitutional".