Greater London
Greater London is a conurbation, a city made from merging many smaller villages, towns and cities into a larger one.  "London" is technically only the City of London, one of London's 33 administrative districts. In order to avoid confusion, "Greater London" is often used to mean the whole modern urban area centred on the City of London, and not just the City of London itself.  When most people say London, they are referring to Greater London.