A curfew can be one of the following:

1. An order by the government for certain persons to return home before a certain time. It can either be to maintain public order (such as that after the 2003 North America blackout), or to suppress targeted groups (such as the one Adolf Hitler enacted on Jewish people in Nazi Germany). Curfews have long been directed at certain groups in many cities or states, such as Japanese-American university students on the West Coast during World War II, African-Americans in many towns during the time of Jim Crow laws, or people younger than 16, 17, 18, 19, 20, 21, or 22 years or another certain age in many towns of the U.S. since the 1980s. Some jurisdictions have also proposed "daytime curfews" that would prevent high school-age youth from visiting public places during school hours or even during immediate after-school hours.
2. An "order" by a person, usually a parent, upon someone, usually under 19, to return home before a certain time, usually from a party or such. This is less formal, but more common.
3. In baseball, a time after which a game must end, or play be suspended. For example, in the American League the curfew rule for many years decreed that no inning could begin after 1 A.M. local time.
4. LaGuardia Airport in New York City operates a rule that after a certain hour, incoming flights must be redirected to Newark Liberty International in New Jersey due to noise restrictions.

The word comes from Anglo-Norman via Middle English, originally an instruction to cover and damp down the fires before retiring, "couvre feu"; a very necessary precaution when cities were filled with wooden houses having thatched roofs.