The head of state was the highest-ranking member of a government. Emperors, kings or queens and presidents ususally served as head of state of their respective polities.
In 2364, William T. Riker defended his choice to adorn himself in the native attire for males on Angel I because of diplomacy. According to Riker, "I have requested an audience with a head of state, and I will honour her by wearing indigenous apparel." (TNG: "Angel One")
Salia was the head of state of Daled IV by 2365. (TNG: "The Dauphin")
Acost Jared was the Ventaxian head of state in 2367. (TNG: "Devil's Due")
In 2373, Kira Nerys requested that Tekeny Ghemor be given the privileges of a visiting head of state during his stay on Deep Space 9. (DS9: "Ties of Blood and Water")