Matthew Poole and continuators on Acts 25:21

21 But when Paul had appealed to be reserved unto the hearing of Augustus, I commanded him to be kept till I might send him to Cesar.

The Emperor who now reigned, and to whom Paul appealed, was Nero, who was called Augustus; this Title being at first appropriated to Octavius, who succeeded Iulius Cesar; but out of honour unto him, or because of its signification, it became an Appellative, and was given unto all the Emperors successively: Nay▪ the Emperor of Germany to this day is called Semper Augustus.

As from Octavius, the Emperors of Rome had the Name of Augustus, so from the first Emperor Iulius, they have the Name of Cesars. This word Cesar, which was the proper Name of the first Emperor, is, in acknowledgment of him, made an Appellative to all his Successors.

Marginal references: Or, Iudgment.