London is the capital and largest city of England and the United Kingdom. It stands on the River Thames in south-east England at the head of a 50-mile (80 km) estuary leading to the North Sea. London has been a major settlement for two millennia. The City of London, its ancient core and financial centre, was founded by the Romans as Londinium. Known colloquially as the Square Mile, it retains boundaries that closely follow its medieval limits. The adjacent City of Westminster has for centuries housed the national government. London has 31 additional boroughs north and south of the river. The London region is governed by the mayor of London and the London Assembly.
London is one of the world's major global cities. It exerts strong influence on the arts, commerce, education, entertainment, fashion, finance, health care, media, professional services, research and development, tourism and transportation. With a GDP of €801.66 billion in 2017, London is the biggest urban economy in Europe. It is one of the largest financial centres in the world and in 2019 had the second highest number of ultra high-net-worth individuals in Europe after Paris. In 2020 London had the second-highest number of billionaires of any city in Europe, after Moscow. London's universities form Europe's largest concentration of higher education institutions. Its high-ranking institutions include Imperial College London in natural and applied sciences, the London School of Economics in social sciences, and the comprehensive University College London. In 2012, London became the first city to have hosted three modern Summer Olympic Games.
London's diverse range of people and cultures mean that more than 300 languages are spoken. Its mid-2018 population (corresponding to Greater London) was about 9 million, which made it Europe's third-most populous city. London accounts for 13.4% of the UK population. Greater London Built-up Area is the fourth-most populous in Europe, after Istanbul, Moscow, and Paris, with 9,787,426 inhabitants at the 2011 census. The London metropolitan area is the third-most populous in Europe, after Istanbul and the Moscow Metropolitan Area, with 14,040,163 inhabitants in 2016.
London has four World Heritage Sites: the Tower of London; Kew Gardens; the Palace of Westminster along with Westminster Abbey, and St Margaret's Church; and the historic settlement in Greenwich, where the Royal Observatory, Greenwich defines the Prime Meridian (0° longitude) and Greenwich Mean Time. Other landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square and The Shard. London has numerous museums, galleries, libraries and sporting events. They include the British Museum, National Gallery, Natural History Museum, Tate Modern, British Library and West End theatres. The London Underground is the oldest rapid transit system in the world.