free city

noun2

1. •
   (historical) Any of various types of city of Europe from classical to modern times, characterised by relative autonomy and/or being responsible only and directly to a king or emperor; a more modern city of such type, sometimes elsewhere than in Europe.

   • Pliny calls ᵉ Tarsus a free city: which is an incontestable proof it was not a Roman colony, for then he would have called it so. — 1727
2. •
   A special economic zone.