The principate refers to a period of time within roman history during which indviduals were no longer required to hold an official office in order to establish power over the Roman Empire. An estimation of this period is given between 27-285 AD, and is often consider the "early empire".
The term Princeps was applied to the person holding the position of power, Augustus was the first of these individuals and his office was derived from personal authority (specifically clientalla bonds, kinship with the Roman Senate, and the dignitas of being related to Julius Caesar). Loosely, the term princeps translates to, "top man", because they did not have an office, however they did hold constitutional power. The equivalent power of the Princeps would be considered an Emperor, however no true office was established.