Sofia (Bulgarian: София, pronounced [ˈsɔfija] ( listen)) is the capital and largest city of Bulgaria and the 12th largest city by population in the European Union, with 1.4 million people living in the Capital Municipality. It is located in western Bulgaria, at the foot of Mount Vitosha, and is the administrative, cultural, economic, and educational centre of the country.

Prehistoric settlements were excavated in the centre of the present city, near the royal palace, as well as in outer districts such as Slatina and Obelya.[citation needed] The well-preserved town walls (especially their substructures) from antiquity date back before the 7th century BC, when Thracians established their city next to the most important and highly respected mineral spring, still functioning today. Sofia has had several names in the different periods of its existence, and remnants from the city's past can still be seen today alongside modern landmarks.

Sofia was first mentioned in the sources as Serdica in relation to Marcus Licinius Crassus' campaigns in 29 BC. The name Serdica or Sardica (Σερδική, Σαρδική) was popular in Latin, Ancient Greek and Byzantine Greek sources from Antiquity and the Middle Ages; it was related to the local Celtic tribe of the Serdi. The name was last used in the 19th century in a Bulgarian text, Service and hagiography of Saint George the New of Sofia: ВЪ САРДАКІИ. Another of Sofia's names, Triaditsa (Τριάδιτζα), was mentioned in Greek medieval sources.

The Bulgarian name Sredets (СРѢДЄЦЪ), an adaptation of Serdica, first appeared in the 11th-century Vision of Daniel and was widely used in the Middle Ages. The current name Sofia was first used in the 14th-century Vitosha Charter of Bulgarian tsar Ivan Shishman or in a Ragusan merchant's notes of 1376; it refers to the famous Hagia Sophia Church, an ancient church in the city named after the Christian concept of the Holy Wisdom. Although Sredets remained in use until the late 18th century, Sofia gradually overcame the Slavic name in popularity. During the Ottoman rule it was called Sofya by the Turkish population.

The city's name is pronounced by Bulgarians with a stress on the 'o', in contrast with the tendency of foreigners to place the stress on 'i'. Interestingly, the female given name "Sofia" is pronounced by Bulgarians with a stress on the 'i'.

Sofia's development as a significant settlement owes much to its central position in the Balkans. It is situated in western Bulgaria, at the northern foot of the Vitosha mountain, in the Sofia Valley that is surrounded by mountains on all sides. The valley is the largest one in the country with territory of 1,186 square kilometres (458 sq mi) and average altitude of 550 metres (1,804 ft). Three mountain passes lead to the city, which have been key roads since antiquity, connecting the Adriatic Sea and Central Europe with the Black and Aegean Seas.

A number of low rivers cross the city, including the Vladaiska and the Perlovska. The Iskar River in its upper course flows near eastern Sofia. The city is known for its numerous mineral and thermal springs. Artificial and dam lakes were built in the last century.

It is located 130 kilometres (81 mi) northwest of Plovdiv,[6] Bulgaria's second largest city, 340 kilometres (211 mi) west of Burgas[6]380 kilometres (236 mi) west of Varna,[6] Bulgaria's major port-cities on the Bulgarian Black Sea Coast. The city is situated at less than 200 kilometres (124 mi) from the borders with three countries: 55 kilometres (34 mi) from Kalotina on the Serbian border, 113 kilometres (70 mi) from Gyueshevo on the frontier with the Republic of Macedonia and 183 kilometres (114 mi) from the Greek border at Kulata.