An oppidum (plural oppida) is a large defended Iron Age settlement. They emerged in the 2nd and 1st centuries BC and spread across Europe, stretching from Britain in the west to the edge of the Hungarian plain in the east. They continued in use until the Romans began conquering Europe. North of the River Danube, where the population remained independent from Rome, oppida continued to be used into the 1st century.

Oppida is a Latin word meaning the main settlement in any administrative area of ancient Rome. The word is derived from the earlier Latin ob-pedum, "enclosed space," possibly from the Proto-Indo-European *pedóm-, "occupied space" or "footprint." Julius Caesar described the larger Celtic Iron Age settlements he encountered in Gaul as oppida, and the term is now used to describe the large pre-Roman towns that existed all across western and central Europe.

In archaeology and pre-history, the term oppida refers to a category of settlement; it was first used in this sense by Reinecke, Dechelette and Dehn in reference to Bibracte, Manching, and Závist. Most definitions of oppida emphasise the presence of fortifications, so they are different from undefended farms or settlements; and urban characteristics, marking them as separate from hill forts. They originated in the 2nd and 1st centuries BC, a product of Europe's La Tène culture and a notional minimum size of 20 to 25 hectares (49 to 62 acres) has often been suggested, although it is flexible and fortified sites as small as 2 hectares (4.9 acres) have been described as oppida. However, the term is not always rigorously used, and has been used to refer to any hill fort dating from the La Tène period. One of the effects of this inconsistency in definitions is that it is uncertain how many oppida were built.