Frankfurt (officially: Frankfurt am Main ( Hessian: Frangford am Maa; lit. "Frank ford on the Main")) is a metropolis and the largest city of the German federal state of Hesse, and its 746,878 (2017) inhabitants make it the fifth-largest city in Germany. On the River Main (a tributary of the Rhine), it forms a continuous conurbation with the neighbouring city of Offenbach am Main, and its urban area has a population of 2.3 million. The city is at the centre of the larger Rhine-Main Metropolitan Region, which has a population of 5.5 million and is Germany's second-largest metropolitan region after the Rhine-Ruhr Region. Since the enlargement of the European Union in 2013, the geographic centre of the EU is about 40 km (25 mi) to the east of Frankfurt's central business district. Like France and Franconia, the city is named after the Franks. Frankfurt is the largest city in the Rhine Franconian dialect area (Franconian dialects).