Ctesiphon (/ˈtɛsɪfɒn/ TESS-if-on; Middle Persian: 𐭲𐭩𐭮𐭯𐭥𐭭, Tyspwn or Tysfwn; Persian: تیسفون; Ancient Greek: Κτησιφῶν, Attic Greek: [ktɛːsipʰɔ̂ːn]; Syriac: ܩܛܝܣܦܘܢ) was an ancient city in modern Iraq, on the eastern bank of the Tigris, about 35 kilometres (22 mi) southeast of Baghdad. Ctesiphon served as a royal capital of the Iranian empires for over eight hundred years, in the Parthian and Sasanian periods. Ctesiphon was the administrative capital of the Sasanian Empire from 226 to 637 (when it was conquered by the Arabs), or until the conclusion of the Muslim conquest of Persia in 651 AD.
Ctesiphon developed into a rich commercial metropolis, merging with the surrounding cities along both shores of the river, including the Hellenistic city of Seleucia. Ctesiphon and its environs were therefore sometimes referred to as "The Cities" (Mahuza, Arabic: المدائن, romanized: al-Mada'in). In the late 6th and early 7th century, it was listed as the largest city in the world by some accounts.