London is, by far, the largest urban area in England and one of the largest and busiest cities in the world. Other cities, mainly in central and northern England, are of substantial size and influence. The list of England's largest cities or urban areas is open to debate because, although the normal meaning of city is "a continuously built-up urban area", this can be hard to define, particularly because administrative areas in England often do not correspond with the limits of urban development, and many towns and cities have, over the centuries, grown to form complex urban agglomerations. For the official definition of a UK (and therefore English) city, see "City status in the United Kingdom".