1) Supreme power or office 
The Principate (27 BC – 284 AD) is the first period of the Roman Empire, extending from the beginning of the reign of Augustus Caesar to the Crisis of the Third Century, after which it was replaced with the Dominate. The Principate is characterized by a concerted effort on the part of the Emperors to preserve the illusion of the formal continuan...
• (n.) Principality; supreme rule.
(from the article `Roman Republic and Empire`) Augustus established a form of government known as a principate, which combined some elements from the republic with the traditional powers of a ... first Roman emperor, following the republic, which had been finally destroyed by the dictatorship of Julius Caesar, his great-uncle and adopti...
In ancient Rome, an unofficial title for the rule of Augustus and his successors, designating the emperor as the leading citizen. ...
The period from the reign of Augustus to the end of the 3rd century AD. 
Prin'ci·pate noun [ Latin principatus : confer French principat .] Principality; supreme rule. [ Obsolete] Barrow. 
supreme power or office.