Greater Istanbul Megaplex or Eastern Marmara Megaplex refers to the multiple metropolitan areas around Istanbul which include the following major cities in Turkey: Tekirdağ, Istanbul, Izmit, Adapazarı, Yalova, Bursa. These metropolitan areas are virtually contiguous; some small gaps (at most half a mile) intervene; and the definition of metroplex, as there is no consensus on it, is loose here. Around 20 million people live in this area, producing about half of Turkey's GDP. This area basically surrounds the eastern part of the Marmara Sea.
References
Greater Istanbul Megaplex Wikipedia(Text) CC BY-SA