What city is called the third Rome?

Moscow was called the Third Rome in the Middle Ages. It was a whole concept, the author of which was the monk Philotheus. The young united Russian state and its ruler needed to legalize their power, explain to the common people why it was Ivan III who should rule all the Russian lands. Therefore, this concept showed the continuity of great empires: the first Rome fell under the onslaught of the barbarians, the second (Constantinople) was captured by the Gentiles, and the third – Moscow, which should stand forever.