An oppidum (pl.: oppida) is a large fortified Iron Age settlement or town. Oppida are primarily associated with the Celtic late La Tène culture, emerging during the 2nd and 1st centuries BC, spread across Europe, stretching from Britain and Iberia in the west to the edge of the Hungarian Plain in the east. These settlements continued to be used until the Romans conquered Southern and Western Europe. Many subsequently became Roman-era towns and cities, whilst others were abandoned. In regions north of the rivers Danube and Rhine, such as most of Germania, where the populations remained independent from Rome, oppida continued to be used into the 1st century AD.

Oppidum is a Latin word meaning 'defended (fortified) administrative centre or town', originally used in reference to non-Roman towns as well as provincial towns under Roman control. The word is derived from the earlier Latin ob-pedum, 'enclosed space', possibly from the Proto-Indo-European *pedóm-, 'occupied space' or 'footprint'. In modern archaeological usage oppidum is a conventional term for large fortified settlements associated with the Celtic La Tène culture.

In his Commentarii de Bello Gallico, Julius Caesar described the larger Celtic Iron Age settlements he encountered in Gaul during the Gallic Wars in 58 to 52 BC as oppida. Although he did not explicitly define what features qualified a settlement to be called an oppidum, the main requirements emerge. They were important economic sites, places where goods were produced, stored and traded, and sometimes Roman merchants had settled and the Roman legions could obtain supplies. They were also political centres, the seat of authorities who made decisions that affected large numbers of people, such as the appointment of Vercingetorix as head of the Gallic revolt in 52 BC.

Caesar named 28 oppida. By 2011, only 21 of these had been positively identified by historians and archaeologists: either there was a traceable similarity between the Latin and the modern name of the locality (e.g. Civitas Aurelianorum-Orléans), or excavations had provided the necessary evidence (e.g. Alesia). Most of the places that Caesar called oppida were city-sized fortified settlements. However, Geneva, for example, was referred to as an oppidum, but no fortifications dating to this period have yet been discovered there. Caesar also refers to 20 oppida of the Bituriges and 12 of the Helvetii, twice the number of fortified settlements of these groups known today. That implies that Caesar likely counted some unfortified settlements as oppida. A similar ambiguity is in evidence in writing by the Roman historian Livy, who also used the word for both fortified and unfortified settlements.

In his work Geographia, Ptolemy listed the coordinates of many Celtic settlements. However, research has shown many of the localisations of Ptolemy to be erroneous, making the identification of any modern location with the names he listed highly uncertain and speculative. An exception to that is the oppidum of Brenodurum at Bern, which was confirmed by an archaeological discovery.

In archaeology and prehistory, the term oppida now refers to a category of settlement; it was first used in that sense by Paul Reinecke, Joseph Déchelette and Wolfgang Dehn [de] in reference to Bibracte, Manching, and Závist. In particular, Dehn suggested defining an oppidum by four criteria:

In current usage, 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 are often described as 'the first cities north of the Alps', though earlier examples of urbanism in temperate Europe are also known. The 2nd and 1st centuries BC places them in the period known as La Tène. A notional minimum size of 15 to 25 hectares (37 to 62 acres) has often been suggested, but that 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 it has been used to refer to any hill fort or circular rampart dating from the La Tène period. One of the effects of the inconsistency in definitions is that it is uncertain how many oppida were built.

In European archaeology, the term oppida is also used more widely to characterize any fortified prehistoric settlement. For example, significantly older hill-top structures like the one at Glauberg (6th or 5th century BC) have been called oppida.