sanctuary citynoun [ C ]
uk/ˈsæŋk.tʃʊə.ri ˌsɪt.i//ˈsæŋk.tʃu.er.i ˌsɪt̬.i/a city in the US or Canada where illegal immigrants (= people who have moved to live in a foreign country permanently) are not prosecuted:
Houston is one of the better-known sanctuary cities for immigrants in the US.