İzmit, known as Nicomedia in antiquity, is a city in Turkey, the administrative center of the Kocaeli Province as well as the Metropolitan Municipality. It is located at the Gulf of İzmit in the Sea of Marmara, about 100 km (62 mi) east of Istanbul, on the northwestern part of Anatolia. The city center has a population of 300,611 (2011 census). The population of the province (including rural areas) is 1,459,772. Unlike other provinces in Turkey, apart from Istanbul, the whole province is included within the municipality of the metropolitan center.
Nicomedia was the eastern and most senior capital city of the Roman Empire between 286 and 324, during the Tetrarchy introduced by Diocletian. Following Constantine the Great's victory over co-emperor Licinius at the Battle of Chrysopolis in 324, Nicomedia served as an interim capital city for Nova Roma.
İzmit derives from the Ancient Greek name of the city, Nicomedia (Greek: Νικομήδεια), prefixed with εις 'to' (similarly to İstanbul). Names used in English prior to official Turkish Latinization include Ismid, Iskimid, and Isnikmid.