Is it ensure, insure, or assure?
Bulgarian capital, Roman Serdica, from the Thracian Serdi people who lived thereabouts. Conquered by the Bulgarians 9c. who altered the name by folk-etymology to Sredeti, which in their tongue meant "center, middle." It got its current name 14c. when the Turks conquered it and converted the 6c. church of St. Sophia into a mosque; the name thence extended to the whole city.
Medieval Latin, from Bulgari "Bulgarians," perhaps literally "the men from the Bolg," the River Volga, upon whose banks they lived until 6c. But the people's name for themselves in Old Bulgarian was Blugarinu, according to OED, which suggests a different origin. In other sources [e.g. Room], the name is said to be ultimately from Turkic bulga "mixed," in reference to the nature of this people of Turko-Finnish extraction but Slavic language.
Republic in southeastern Europe in the eastern part of the Balkan Peninsula, bordered by Romania to the north, the Black Sea to the east, Turkey to the southeast, Greece to the south, and Macedonia and Yugoslavia to the west. Its capital and largest city is Sofia.
Note: Former Eastern Bloc country. Soviet troops entered Bulgaria in 1944, and a communist government was established soon thereafter. Bulgaria's communist rulers followed the Soviet lead for almost fifty years, until the collapse of the Soviet Union. In January 1991, a multiparty government began to institute democratic and economic reforms.