borough (district), administrative and historic county of Kent, England, on the River Medway southeast of London. It lies to the south of the North Downs and northeast of the Weald in west central Kent. Although parts of the borough remain rural, it has also developed as a residential area for London commuters. The main urban area is Tonbridge, an old market town at the southern edge of the district. Area 93 square miles (240 square km). Pop. (2001) 107,560.