Robert Waldmann inquires as to the meaning of the Latin phrase: "arcana imperii."
It is a reference to a famous passage about the Year of the Four Emperors in the Historiae of Publius (or maybe Gaius: I don't think we are sure) Cornelius Tacitus, Replacement Consul in the year 97 (during the reign of the Emperor Nerva) and Governor of Asia Province sometime later (during the reign of the Emperor Trajan):
Evolgato imperii arcano, posse principem alibi quam Romae fieri (a piece of the hidden knowledge of power-holding was revealed: making an emperor could be at a place elsewhere than Rome).
"Arcanus" in Tacitus means something hidden, something mystical, something complex and subtle, something wise, something dangerous, something that certainly should not be well-known among the general public.
"Imperium" means the sphere of power and legitimate authority of a Roman official--by the time Tacitus was writing there was only one official who mattered, the Emperor, and his sphere of power and legitimate authority was the entire Roman Empire.
: Yes, the English translation of "alibi" is "alibi": the demonstration that you were elsewhere when the crime was going down...