It is an ancient name, attested already in the first century AD, usually in the Latinised form Londinium; for example, handwritten Roman tablets recovered in the city originating from AD 65/70-80 include the word Londinio ("in London").
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 Common Brythonic; recent work tends to reconstruct the lost Celtic form of the name as *[Londonjon] or something similar. This was adapted into Latin as Londinium and borrowed into Old English, the ancestor-language of English.
The toponymy of the Common Brythonic 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)'.
Until 1889, the name "London" applied to the City of London, but since then it has also referred to the County of London and Greater London. "London" is sometimes written informally as "LDN".