district, administrative county of West Sussex, historic county of Sussex, England. It is named for the River Adur, which cuts through the chalk ridge of the South Downs via an impressive water gap before entering the English Channel at Shoreham-by-Sea. The district is small and occupies the lower valley of the river between the urban areas of Worthing to the west and Brighton-Hove to the east. Area 16 square miles (42 square km). Pop. (2001) 59,625.