Wikipedia
Samnium is a Latin exonym for a region of Southern Italy anciently inhabited by the Samnites. Their own endonyms were Safinim for the country (attested in one inscription and one coin legend) and Safineis for the The language of these endonyms and of the population was the Oscan language. However, not all the Samnites spoke Oscan, and not all the Oscan-speakers lived in Samnium.
The ancient geographers were unable to relay a precise definition of Samnium's borders. Moreover, the areas it included vary depending on the time period considered. The main configurations are the borders it had during the floruit of the Oscan speakers, from about 600 BC to about 290 BC, when it was finally absorbed by the Roman Republic.
This originary Samnium should not be confused with the later territory of the same name. Rome's first Emperor, Augustus, divided Italy into 11 regions. Although these entities only served administrative purposes, and were identified with the sole numeral, by scholarly convention the Regio IV has been dubbed "Samnium". Ancient Samnium had actually been split up into three of the Augustan regions.
The name survives in Italian today, but it defines only a small portion of what it once was - virtually, only the Province of Benevento.