borough (district), administrative and historic county of Surrey, southeastern England immediately south of Greater London. Named after the two principal locales of the district, Reigate and Banstead extends across the North Downs, a range of low chalk hills trending east–west. The district is residential and yet contains extensive areas of open space and parkland; recreational and outdoor sports facilities are commonplace throughout. Diverse industrial estates produce chemicals, leather, and pharmaceuticals. Man-made tunnels probably associated with a medieval castle at Reigate, the borough seat, were used as air raid shelters and for storage during World War II. Area 50 square miles (129 sq km). Pop. (2001) 126,519.