Akershus Fortress in the context of "Oslo"

59°54′19.68″N 10°45′35.05″E / 59.9054667°N 10.7597361°E / 59.9054667; 10.7597361Aker was a former independent municipality in Akershus, Norway, that constitutes the vast majority of the territory of the modern city of Oslo.

The name originally belonged to a farm which was located near the current Old Aker Church. The church in turn became the source of the name of the parish and later municipality as well as Akershus Fortress, the main fief and main county of Akershus which included most of Eastern Norway until 1919, the smaller county of Akershus, and numerous institutions within this area.

Medieval Oslo refers to the urban community of Oslo during the Middle Ages and up until the city fire of 1624, when the town burned to the ground and was not rebuilt on its original site. Instead, the city of Christiania was founded nearby. After this, the original name "Oslo" continued to be used for the original, burned-down area, which was largely converted into farmland. In 1925, the municipality of Christiania changed its name to Oslo, meaning that in modern times, "Oslo" refers to a much larger area than the old medieval town.

The ruins of the medieval town, made of stone and brick, are located in the area now called Gamlebyen ("The Old Town"), roughly bordered by the Hovinbekken stream in the north, the original course of the Alna River to the south and east, and the water features associated with the Medieval Park in the west. The Franciscan monastery (partly overbuilt by Gamlebyen Church and Oslo Hospital) just east of the Alna is also considered part of the Medieval Town, despite being located on the "back side" of the river. Akershus Castle and Fortress (begun in 1297) and Hovedøya with its Cistercian monastery also belong to the Medieval Town.

Akershus (Norwegian pronunciation: [ɑkəʂˈhʉːs] ) is a county in Norway, with Oslo as its administrative centre, though Oslo is not located within Akershus. Akershus has been a region in Eastern Norway with Oslo as its main city since the Middle Ages, and is named after the Akershus Fortress in Oslo and ultimately after the medieval farm Aker in Oslo. From the Middle Ages to 1919, Akershus was a main fief and main county that included most of Eastern Norway, and from the 17th century until 2020 and again from 2024, Akershus also has a more narrow meaning as a smaller central county in the Greater Oslo Region. Akershus is Norway's largest county by population with over 716,000 inhabitants.

Originally Akershus was one of four main fiefs in Norway and included almost all of Eastern Norway. The original Akershus became a main county (Stiftamt or Stift) in 1662 and was sometimes also known as Christiania Stift. It included several subcounties (Amt or Underamt); in 1682 its most central areas, consisting of modern Oslo and Akershus, became the subcounty of Akershus within the larger main county of the same name. In 1842, the capital city of Christiania, which at the time consisted of a tiny part of modern Oslo, became a separate subcounty within Akershus main county. The main county of Akershus was disestablished in 1919, and the subcounty continued as Akershus county (fylke). During its history Akershus (sub) county ceded territory to Oslo several times; Akershus' most central and important municipality, Aker, was transferred to Oslo in 1948. Thus, while modern Akershus' capital is Oslo, Oslo is not located within the modern county itself. In 2020, the county of Akershus was merged into Viken along with the counties of Østfold and Buskerud, but Akershus was reestablished as a county from 2024 with slightly enlarged borders. Modern Akershus borders Oslo, Hedmark, Oppland, Buskerud, Oslo, and Østfold; it also has a short border with Sweden (Värmland).