Means first citizen and is the root of our word prince.
Caesar Augustus was a king, and his senate merely felt as part of the government. He sent a message to the Roman world by calling himself Princeps. It's an old Roman Republican term - someone old and revered would be called a first citizen, a point of honor he brought upon himself. From the rule of Caesar Augustus to Emperor Diocletian is called the Principate, the era of the rule of the emperors of the first citizens.
(Hillsdale College - Ancient Christianity)