Excerpts from Books and Wikipedia
Octavian, the great nephew and adoptive heir of Julius Caesar, was 18 at the time of Caesar's assassination. He later shared power with Mark Antony and Lepidus. He defeated Mark Antony at Actium in -31, becoming the sole ruler of Rome. Octavian went to the Senate in -27 to lay down his powers. Instead, he was made first citizen (Princeps) and illustrious one (Augustus). "Octavian was emperor in all but name, and his rule marks the beginning of the Roman Empire." [Furtado: 1001 Days]