To translate what Cowboy said into understandable English: Greater London is an administrative unit, which is larger than the City of London itself. Hence the name Greater London. There is also a Greater Manchester for the very same reason.

Greater London is the top-level administrative subdivision covering London, England.The administrative area was officially created in 1965 and covers the City of London, including Middle Temple and Inner Temple, and the 32 London boroughs (including the City of Westminster).Its area also forms the London region of England and the London European Parliament constituency. The region has by far the highest GDP per capita in the United Kingdom. It covers 1572 km2 (607 square miles) and had a 2006 mid-year estimated population of 7,512,400.The term Greater London was in use before 1965 to refer to a variously defined area, larger than the County of London and often similar to the Metropolitan Police District. cowboy...

In simple terms, it's the London Metropolitan area.