Which period of Roman history was referred to as the Principate?

The term Principate was sometimes given to the early part of the Roman Empire beginning with the reign of Augustus Caesar in 27 BC. The rule of a single emperor ("princeps") was masked by using the title "first among the senators / first among the citizens" ("princeps senatus / princeps civitatis"). The cover was to make the emperor not appear as a king-like ruler, but more like the senior member of the Roman Senate.

The Principate period lasted until the "Crisis of the Third Century" in 284 AD, a period of Roman history that saw the near collapse of the empire. The term "dominus" (lord, master) was used by Emperor Diocletian which had a more royal connotation, sometimes referred to as the beginning of Dominate period. Following the crisis, control of the empire was divided into parts known as the tetrarchy with two senior "Augusti" and two junior "Caesars" instead of a single princeps. This division led to the eventual split between the western and eastern portions of the empire, with the western empire falling in 476 AD and the eastern (known later as the Byzantine Empire) falling in 1453.