Literally, oppidum just means a town. Caesar used the word more specifically to describe the large fortified native sites in Britain and Gaul. It had no precise legal meaning or status within the empire.
Whilst the definition remains far from precise, the word is used similarly today to describe large late Iron Age enclosed settlements usually of high status and often regarded as major tribal centres, such as Stanwick in North Yorkshire. The term ‘Territorial Oppidum’ is sometimes used to describe large tracts of land clearly enclosed as one unit, such as the dykes at Chichester which enclosed at least 150 sq km.
An introductory guide to oppida in Britain is available from Historic England at
https://content.historicengland.org.uk/images-
Oppidum