Pannonia (pəˈnoʊniə, panˈnɔnia) was a province of the Roman Empire bounded on the north and east by the Danube, coterminous westward with Noricum and upper Italy, and southward with Dalmatia and upper Moesia. Pannonia was located in the territory that is now western Hungary, western Slovakia, eastern Austria, northern Croatia, north-western Serbia, northern Slovenia, and northern Bosnia and Herzegovina.
Julius Pokorny believed the name Pannonia is derived from Illyrian, from the Proto-Indo-European root *pen-, "swamp, water, wet" (cf. English fen, "marsh"; Hindi pani, "water").
Pliny the Elder, in Natural History, places the eastern regions of the Hercynium jugum, the "Hercynian mountain chain", in Pannonia and Dacia (now Romania). He also gives us some dramaticised description of its composition, in which the proximity of the forest trees causes competitive struggle among them (inter se rixantes). He mentions its gigantic oaks. But even he—if the passage in question is not an interpolated marginal gloss—is subject to the legends of the gloomy forest. He mentions unusual birds, which have feathers that "shine like fires at night". Medieval bestiaries named these birds the Ercinee. The impenetrable nature of the Hercynia Silva hindered the last concerted Roman foray into the forest, by Drusus, during 12–9 BC: Florus asserts that Drusus invisum atque inaccessum in id tempus Hercynium saltum (Hercynia saltus, the "Hercynian ravine-land") patefecit.
The Illyrians (Illyrii) were the first known people to inhabit the region. Their immigration took place during the late Bronze Age and the early Iron Age. The leading class of the indigenous population was expelled or killed, while the majority of the population was obliged to pay tax. The locals were massacred in only some places, so the old crafts and traditions endured. The culture of the region changed with the arrival of more Illyrian warriors, who even engaged in conflicts with the earlier arrived Illyrians. The Illyrians in Pannonia were distinguished by the Romans as Pannonians (Pannonii).