The first emperor bequeathed the title Augustus to his adopted heir and successor Tiberius in his will.[4] From then on, though it conferred no specific legal powers, Augustus was a titular element of the imperial name.[4] Subsequently, the title was bestowed by the Roman Senate.[4] Until the reign of Marcus Aurelius (r.?161–180), the title was unique to its bearer; in 161 Marcus Aurelius elevated Lucius Verus (r.?161–169) to Augustus and both bore the title at the same time.[4]
The date of an emperor's investiture with the title Augustus was celebrated as the dies imperii and commemorated annually.[4] From the 3rd century, new emperors were often acclaimed as Augusti by the army.[4] Emperors also inherited Caesar (originally a family name) as part of their titles.
In addition, the Senate awarded him the appellation of augustus ("elevated"). The honorific itself held no legal meaning, but it denoted that Octavian (henceforth Augustus) now approached divinity, and its adoption by his successors made it the de facto main title of the emperor. He also received the civic crown alongside several other insignias in his honor. Augustus now held supreme and indisputable power, and even though he still received subsequent
grants of powers, such as the granting of tribunicia potestas in 23 BC, these were only ratifications of the powers he already possessed.[10]
Augustus actively prepared his adopted son Tiberius to be his successor and pleaded his case to the Senate for inheritance on merit. After Augustus' death in AD 14, the Senate confirmed Tiberius as princeps and proclaimed him as the new augustus. Tiberius had already received imperium maius and tribunicia potestas in AD 4, becoming legally equal to Augustus but still subordinate to him in practice.[13] The "imperial office" was thus not truly defined until the accession of Caligula, when all of Tiberius' powers were automatically transferred to him as a single, abstract position that was symbolized by his sacred title of augustus.[14]
The weakest point of the Augustan institution was its lack of a clear succession system.[35] Formally announcing a successor would have revealed Augustus as a monarch, so he and subsequent emperors opted to adopt their best candidates as their sons and heirs.