What is Curfew?

Legal Definition
The name of a law, established during the reign of the English king, William, the conquerer, by which the people were commanded to dispense with fire and candle at eight o'clock at night. It was abolished in the reign of Henry I., but afterwards it signified the time at which the curfew formerly took place. The word curfew is derived, probably, from couvre few, or cover fire. 4 Bl. Com. 419, 420.
-- Bouviers Law Dictionary
Legal Definition
An institution supposed to have been introduced into England by order of William the Conqueror, which consisted in the ringing of a bell or bells at eight o'clock at night, at which signal the people were required to extinguish all lights in their dwellings, and to put out or rake up their fires, and retire to rest, and all companies to disperse. The word is probably derived from the French couvre feu, to cover the fire.
-- Black's Law Dictionary
Legal Definition
A curfew is an order specifying a time during which certain regulations apply.
-- Wikipedia
Legal Definition
A town bell rung in the evening as a signal for extinguishing lights and fires.
-- Ballentine's Law Dictionary