district, administrative county of West Sussex, historic county of Sussex, England. It is named for the River Arun, which rises near the northern boundary of the county and flows south across it. The district includes the ancient borough of Arundel, with the nearby Arundel Castle, built soon after the Norman Conquest (1066) and now held in trust as a residence of the Duke of Norfolk. The district embraces a stretch of the South Downs and a section of the Channel coast that includes a line of holiday resorts, including Littlehampton and Bognor Regis. Area 85 square miles (221 square km). Pop. (2001) 140,787.