Wikipedia
Tsargrad ( Old Church Slavonic: Цѣсарьградъ; Church Slavonic; Царьгра̀дъ, ; South Slavic languages: Carigrad or Цариград, depending on their alphabets (or Tsarigrad as an alternative Latin transliteration of Cyrillic); ; ; ; also rendered as Czargrad and Tzargrad; see Tsar) is a Slavic name for the city or ""land of"" of Constantinople, the capital of the Eastern Roman Empire, commonly known as the Byzantine Empire, and present-day Istanbul in Turkey.
For ; Tsar/tsara "land of" written in today's Romanian letters as (Ts, replaced by Ţ it making same sound as Ts) therefore litterly meant (land for Grad) or country of Grad. This meant (Belgrad the way they say) Belgrade, or Belgrade and surrounding zone of influence in Romanian, Moldavian, Vlachi language, as Referenced in ""Neacșu's letter"". Same rule and meaning apply for Moldavian, Vlachi. Other historic Slavonic names of the city were Константинь градъ (Konstantin grad; in Old Church Slavonic as well as Church Slavonic) and Константиноградъ (Konstantinograd; only in Church Slavonic). Both are direct translation of the Greek name of the city (Κωνσταντινούπολη) and mean "the city of Constantine".
Tsargrad is an Old Church Slavonic translation of the Greek Βασιλὶς Πόλις. Combining the Slavonic words tsar for " Caesar / Emperor" and grad for "city", it stood for "the City of the Caesar". According to Per Thomsen, the Old Russian form influenced an Old Norse appellation of Constantinople, Miklagard (Мikligarðr).
Bulgarians also applied the word to Tarnovgrad (Tsarevgrad Tarnov, "Imperial City of Tarnov"), one of the capitals of the Bulgarian tsars, but after the Balkans fell under Ottoman rule, the Bulgarian word has been used exclusively as another name of Constantinople.
After the fall of Constantinople in 1453, the burgeoning Russian Empire began to see itself as the last extension of the Roman Empire, and the force that would resurrect the lost leviathan ( Third Rome). This belief was the supported by the Russian Orthodox Church and given at least an air of legitimacy by the marriage of Ivan III to the heiress of the last Byzantine Emperor. It was allegedly an objective of the Tsars to recapture the city, but despite many southern advances and expansion by the empire, this was never realized owing to the Western interference in the Crimean War.
As the zeitgeist which spawned the term has faded, the word Tsargrad is now an archaic term in Russian. It is however still used occasionally in Bulgarian, particularly in a historical context. A major traffic artery in Bulgaria's capital Sofia carries the name Tsarigradsko shose (" Tsarigrad Road"); the road begins as the Tsar Osvoboditel Boulevard and continues into the main highway that leads southeast to Istanbul. The name Tsarigrad is also retained in word groups such as tsarigradsko grozde ("Tsarigrad grapes", meaning " gooseberry"), the dish tsarigradski kyuftentsa ("small Tsarigrad koftas") or sayings like "One can even get to Tsarigrad by asking". In Slovene it is still largely used and often preferred over the official name. People also understand and sometimes use the name Carigrad in Bosnia, Croatia, Montenegro and Serbia.