urban district from which the non-metropolitan district was formed in 1974
the following text is based on an article in Wikipedia
Watford is a town and borough in Hertfordshire, England, situated 17 miles (27 km) northwest of central London and inside the circumference of the M25 motorway. It should not be confused with Watford, Northamptonshire which is 55 miles to the north and known for the "Watford Gap".
Watford was created as an urban district under the Local Government Act 1894, and became a municipal borough by grant of a charter in 1922. In 1974 it became the Watford Borough Council (or Watford non-metropolitan District).
Watford District does not appear to cover any other former administrations than Watford Urban District itself. However, there was a number of absorptions of parts of parishes just outside Watford between 1900 and 1974. Places now described as in Watford may well have been in a parish in Watford Rural District at the beginning of the century.