What city does the "Third Rome" concept refer to?

Third Rome refers to the doctrine that Russia or, specifically, Moscow succeeded Rome and Byzantium Rome as the ultimate center of true Christianity and of the Roman Empire. This is the most generally misunderstood and abused of the several expressions of Russia's new place in the world resulting from domestic and international events of the 1430s and 1520s. The monk Filofei of the Pskov-Eliazarov monastery formulated it in one or two epistles, written between 1523 and 1526, which were then reworked during the sixteenth and seventeenth centuries.