Roman Provinces
The empire was divided into provinces (provincia, plural provinciae) led by senatorial statesmen (usually consuls or praetors).
History
Republic of Rome
Provinciae referred to ministerial roles. Some of these roles involved regional governance, but others represented tasking a military commander with conquering a region not yet under imperial rule, and still others were administrative roles like the treasury. Similarly, a region under imperial rule was not necessarily assigned a provinciae. Finally, many provinciae were temporary measures.
In 122 BC, tribune Gaius Sempronius Gracchus pushed reforms requiring that provinciae be selected before consular elections. Because the positions had to be established about 6 months ahead of when a consul would actually be sent, temporary provinciae became ineffective.
Concurrently, the expansion of the empire created more provinciae than could be filled with consuls. It became common for praetors to be granted a promagistrate (pro magistratu), essentially an indefinite prorogation of the post. They either led with the authority of a praetor (pro praetore) or a consul (pro consule).
Altogether, there was a rapid consolidation of power within and across the provinces. This led directly to the fall of the republic, and the rise of the emperor.
Imperial Rome
Augustus personally held most of the provinciae; these became known as imperial provinces. He appointed legates to lead many border provinces as legatus Augusti pro praetore.
In the 290s, Diocletian reorganized the empire into about 100 provinces under twelve dioceses.