The term "Tetrarchy" refers to a system of government established by the Roman Emperor Diocletian in the late 3rd century AD. It divided the Roman Empire into four regions, each ruled by a leader known as a Tetrarch. This system aimed to improve administration and defense against external threats by distributing power among multiple rulers. Under the Tetrarchy, the empire was split into two main parts: the Western Roman Empire and the Eastern Roman Empire. Each part had a senior emperor, called an Augustus, and a junior emperor, known as a Caesar. This structure helped stabilize the empire during a time of crisis and laid the groundwork for future governance.