district in the northern part of the administrative and historic county of Buckinghamshire, southern England. The name comes from the rich clay Vale of Aylesbury, which is situated between the ridges of the Cotswolds and the Chiltern Hills. The district includes both Aylesbury and Buckingham, 16 miles (26 km) farther north, and their surrounding rural countryside. Area 349 square miles (904 square km). Pop. (2001) 165,749.