Bosnia (Serbo-Croatian: Bosna/Босна; pronounced [bɔ̂sna]) is the northern region of Bosnia and Herzegovina, encompassing roughly 81% of the country; the other eponymous region, the southern part, is Herzegovina. Bosnia is an informal term for the whole country.
The two regions have formed a geopolitical entity since medieval times, and the name “Bosnia” commonly occurs in historical and geopolitical senses as generally referring to both regions (Bosnia and Herzegovina). The official use of the combined name started only in the late period of Ottoman-rule.
Bosnia lies mainly in the Dinaric Alps, ranging to the southern borders of the Pannonian plain, with the rivers Sava and Drina marking its northern and eastern borders.
The area of Bosnia comprises approximately 41,000 km2, and makes up about 80% of the territory of the present-day state of Bosnia and Herzegovina. There are no true borders between the region of Bosnia and the region of Herzegovina. Unofficially, Herzegovina is south of the mountain Ivan planina. According to another unofficial definition, Herzegovina encompasses watersheds Neretva and Trebišnjica rivers.