The security chief (or chief of security) is the officer in charge of the Starfleet Security of a starship or starbase.
In the Earth Starfleet, the armory officer was in charge of a ship's security department.
In the 2270s, the role of security chief was combined with that of the Chief Tactical Officer. The Chief Tactical Officer could be a separate position or be combined with that of Chief Helm Officer or navigator.
In the 24th century, with helm and navigation consolidated in the new flight controller position, the chief tactical officer fulfilled the role of security chief.
On starbases, the chief of security is usually a separate position from the tactical officer.