London is a leading global city in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transportation. It is the world's largest financial centre and has the fifth or sixth largest metropolitan area GDP in the world. London is often regarded as a world cultural capital. It is the world's most-visited city as measured by international arrivals and has the world's largest city airport system measured by passenger traffic. It is the world's leading investment destination, hosting more international retailers and ultra high-net-worth individuals than any other city. London's universities form the largest concentration of higher education institutes in Europe. In 2012, London became the first city to have hosted the modern Summer Olympic Games three times.
London has a diverse range of people and cultures, and more than 300 languages are spoken in the region. Its estimated mid-2016 municipal population (corresponding to Greater London) was 8,787,892, the largest of any city in the European Union and accounting for 13.4% of the UK population. London's urban area is the second most populous in the EU, after Paris, with 9,787,426 inhabitants at the 2011 census. The city's metropolitan area is the most populous in the EU with 14,040,163 inhabitants in 2016, while the Greater London Authority states the population of the city-region (covering a large part of the south east) as 22.7 million. London was the world's most populous city from around 1831 to 1925.
London contains four World Heritage Sites: the Tower of London; Kew Gardens; the site comprising the Palace of Westminster, Westminster Abbey, and St Margaret's Church; and the historic settlement of Greenwich (in which the Royal Observatory, Greenwich defines the Prime Meridian, 0° longitude, and GMT). Other landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square and The Shard. London is home to numerous museums, galleries, libraries, sporting events and other cultural institutions, including the British Museum, National Gallery, Natural History Museum, Tate Modern, British Library and West End theatres. The London Underground is the oldest underground railway network in the world.
The etymology of London is uncertain. It is an ancient name, attested already in the first century AD, usually in the Latinised form Londinium; for example, hand-written Roman tablets recovered in the city originating from AD 65/70-80 include the word Londinio ("in London").
Over the years, the name has attracted many mythicising explanations. The earliest attested appears in Geoffrey of Monmouth's Historia Regum Britanniae, written around 1136. This had it that the name originated from a supposed King Lud, who had allegedly taken over the city and named it Kaerlud.
Modern scientific analyses of the name must account for the origins of the different forms found in early sources Latin (usually Londinium), Old English (usually Lunden), and Welsh (usually Llundein), with reference to the known developments over time of sounds in those different languages. It is agreed that the name came into these languages from British Celtic; recent work tends to reconstruct the lost Celtic form of the name as * or something similar. This was adapted into Latin as Londinium and borrowed into West Germanic, the ancestor-language of English, already before English had become widely spoken in Britain.
However, the etymology and original meaning of the British Celtic form is much debated. A prominent explanation was Richard Coates's 1998 argument that the name derived from pre-Celtic Old European *(p)lowonida, meaning "river too wide to ford". Coates suggested that this was a name given to the part of the River Thames which flows through London; from this, the settlement gained the Celtic form of its name, *Lowonidonjon. However, most work has accepted a Celtic origin for the name, and recent studies have favoured an explanation along the lines of a Celtic derivative of an proto-Indo-European root *lendh- ('sink, cause to sink'), combined with the Celtic suffix *-injo- or *-onjo- (used to form place-names). Peter Schrijver has specifically suggested, on these grounds, that the name originally meant 'place that floods (periodically, tidally)'.