The modern county of Telemark consists of several distinct historical regions. It takes its name from the largest of them, which is now called Upper Telemark, but which was historically simply called Telemark. Telemark is named for the thelir (Þilir in Old Norse), the ancient Germanic tribe that inhabited what is now known as Upper Telemark since the Migration Period and during the Viking Age. The Norse form of the name was Þelamǫrk. The first element is the genitive plural case of þelir while the last element is mǫrk "woodland, borderland, march".
Traditional Telemark, i.e. Upper Telemark, is located in the inland of the modern county, and comprises more than two thirds of it according to its traditional definition. Both in medieval times and later (Upper) Telemark was the region of Norway with the most self-owning farmers. It retained Norse culture to a larger degree than any other region in Norway, with respect to its more egalitarian organisation of society, religion, traditional values and language. Thus the people of Telemark were often described during the middle ages and early modern era as the most violent in Norway. The dialects of Upper Telemark are also the dialects of Norwegian that are closest to Old Norse. The farmers of Telemark were marked by a strong-willed conservatism and belief in their traditional values that often defied the central authorities of Denmark-Norway; for example they held on to aspects of both Old Norse religion and later of Catholicism longer than other regions in Norway. (Upper) Telemark traditionally lacks cities entirely.
Grenland and the Skien fjord are flatter regions located closer to or at the coast. Historically Grenland referred to what is now called Midt-Telemark, but over time the name Grenland has come to refer to the Skien fjord area. The latter is traditionally characterized by its cities and its involvement in seafaring and trade. It also includes several larger agricultural properties and estates, as well as industry. The culture and social structure are more urban, far less traditional, more influenced by contact with continental Europe and far less egalitarian. The most important city of the region, Skien, was historically one of Norway's most important cities, although its importance declined after the Napoleonic Wars. The playwright Henrik Ibsen was a native of Skien, and many of his plays are set in places reminiscent of the city and area.
During the Dano-Norwegian union the traditional regions of Telemark and Grenland/the Skien fjord became the fief (len) and later county (amt) of Bratsberg (Bradsberg). The fief and county was named after the farm Bratsberg (Norse Brattsberg), since this was the seat of the governor. The first element is the genitive case of brattr m "steep mountain", the last element is berg n "mountain". (The name is referring to a steep mountainside behind the farm.) In 1919 Bratsberg county was renamed Telemark. Despite this, Grenland retains a separate identity that is distinct from Telemark proper; the minority in the Storting voted for the name Grenland–Telemark in 1918.
(Upper) Telemark, particularly Kviteseid, is known as the birthplace of skiing as a modern sport. Telemark lent its name to Telemark skiing, a style invented by Sondre Norheim, and the characteristic Telemark landing of ski jumping. Telemark is also known as the centre of the Bunad movement. Telemark has more buildings from medieval times than any other Norwegian region.