The capital of the United Kingdom, in southeastern England on the Thames River; population 7,619,800 (est. 2008). London, called Londinium, was settled as a river port and trading center shortly after the Roman invasion of ad 43 and has been a flourishing center since the Middle Ages. It is divided administratively into the City of London, which is the country's financial center, and 32 boroughs. An industrial city in southwestern Ontario, Canada, north of Lake Erie; population 353,395 (2006).