Peterborough, city and unitary authority, geographic county of Cambridgeshire, England. At the core of the city and unitary authority is a historic region called the Soke of Peterborough, which encompasses the original town of Peterborough and an area extending west between the Rivers Welland and Nene; the Soke of Peterborough is part of the historic county of Northamptonshire. The city and unitary authority also embrace an area to the east, around Thorney, within the historic county of…