In historiography, the "imperial period" is by convention taken to last from 27 BCE to CE 284.
This follows Hans Jürgen Eggers [de] (1955), who used a periodization of "early imperial period" (German: frühkaiserzeitlich) B1 to B2 and "late imperial period" (German: spätkaiserzeitlich) C1 to C3, reflecting the history of Roman pottery imports to Magna Germania and other parts of Barbaricum (Eggers A corresponds to La Tène D).
In the chronology of Eggers (1955):[1] The term "Roman imperial period" has been used as opposed to "late antiquity", i.e. implying the "early" and "middle" imperial period of the late 1st century BC to the 3rd century CE.
The "Roman imperial period" in this sense would end with the reforms under Diocletian and the beginning of the Christianization of the Roman Empire.
The period is roughly equivalent in span to the "Principate", the early period of Roman imperial rule from Augustus to Diocletian (r. 284–305), succeeded by the "Dominate".