Westminster is the name of a city that covers much of central London, located to the west of the ancient City of London, and which has been the principal seat of government in England for more than nine hundred years. Both cities, and much of the surrounding conurbation, are collectively known more generally as London. For the modern administrative area covered by the London Borough of Westminster, see City of Westminster.

The Palace of Westminster came to be the principal royal residence after the Norman Conquest of England in 1066, and later housed the developing Parliament and law courts of England. The neighbouring Westminster Abbey became the traditional venue of the coronation of the kings of England.

The monarchy later moved to other palaces elsewhere in the city, and the law courts have since moved to the Royal Courts of Justice, close to the border of the City of London, so today "Westminster" is often used as shorthand for Parliament and the political community (including the elected Government) of the United Kingdom generally. The civil service is similarly referred to by the area it inhabits, Whitehall, where there was also once a royal palace. "Westminster" is consequently also used in reference to the Westminster System, the parliamentary model of democratic government that has evolved in the United Kingdom. The Westminster System is used with some adaptation in many other nations, particularly in the Commonwealth of Nations and other parts of the former British Empire.

Close to the Palace of Westminster and Westminster Abbey is Westminster School, one of the major English public schools. Three of the four campuses of the University of Westminster are within the borough.