Old Livonia in the context of Bishopric of Dorpat

The State of the Teutonic Order (German: Staat des Deutschen Ordens, pronounced [ˈʃtaːt dɛs ˌdɔʏtʃn̩ ˈʔɔʁdn̩s] ) was a theocratic state located along the southeastern shore of the Baltic Sea in northern Europe. It was formed by the knights of the Teutonic Order during the early 13th century Northern Crusades in the region of Prussia. In 1237, the Livonian Brothers of the Sword merged with the Teutonic Order of Prussia and became known as its branch – the Livonian Order (while their state, Terra Mariana, covering present-day Estonia, Latvia, and a small part of Russia, became part of the State of the Teutonic Order). At its greatest territorial extent during the early 15th century, the State encompassed Chełmno Land, Courland, Gotland, Livonia, Estonia, Neumark, Pomerelia (Gdańsk Pomerania), Prussia and Samogitia.

Following the battles of Grunwald in 1410 and Wilkomierz in 1435, the State fell into decline. After losing extensive territories in the imposed Peace of Thorn in 1466, the extant territory of its Prussian branch became known as Monastic Prussia (Polish: Prusy zakonne) or Teutonic Prussia (Polish: Prusy krzyżackie). It existed until 1525 as a fiefdom of the Polish Crown. The Livonian branch joined the Livonian Confederation and continued to exist as part of it until 1561.

Livonia, known in earlier records as Livland, is a historical region on the eastern shores of the Baltic Sea. It is named after the Livonians, who lived on the shores of present-day Latvia.

By the end of the 13th century, the name was extended to most of present-day Estonia and Latvia, which the Livonian Brothers of the Sword had conquered during the Livonian Crusade (1193–1290). Medieval Livonia, or Terra Mariana, reached its greatest extent after the Saint George's Night Uprising (1343–1345), which forced Denmark to sell the Duchy of Estonia (northern Estonia conquered by Denmark in the 13th century) to the State of the Teutonic Order in 1346. Livonia, as understood after the retreat of Denmark in 1346, bordered on the Gulf of Finland in the north, Lake Peipus and Russia to the east, and Lithuania to the south.

The Bishopric of Ösel–Wiek (Estonian: Saare-Lääne piiskopkond; German: Bistum Ösel–Wiek; Low German: Bisdom Ösel–Wiek; contemporary Latin: Ecclesia Osiliensis) was a Roman Catholic diocese and a semi-independent prince-bishopric — part of Terra Mariana (Old Livonia) in the Holy Roman Empire. The bishopric covered what are now Saare, Hiiu, Lääne counties and the western part of Pärnu county of Estonia.