Late Antiquity is a term that is used for the historical period of the Roman Empire from roughly the chaos of the third century that proceeded Diocletian to the rise of Islam. It spans from Classical Antiquity to Medieval History. It was a period of transition from the residues of the old Republican forms of government to the small kingdoms that dotted the western world for centuries. Diocletian straightened out the Roman Empire, at least temporarily. He was the emperor who voluntarily divided his empire with three other men, two of whom were subordinates, known as Caesares, and the other two the lead emperors [see Tetrarchy Table]. One of the lead emperors was Diocletian. He and the other were known as Augusti. This is roughly the time the masters of Rome stopped trying to maintain illusion of Rome's still being a Republic. This period was known as the Dominate. The emperor was now lord and master of his people.
Read more about the period in this Dominate Timeline.