The Head of State is the individual in a government who serves as the chief representative of that government's state, personifying the continuity and legitimacy of the state. A head of state sometimes has political power and is, in fact, the leader of his or her nation (the head of government); other heads of state are almost purely ceremonial.
Ceremonial heads of state in the modern era have included the posts of Emperor of the Klingon Empire (Kahless) and Emperor of the Romulan Star Empire (Shiarkiek). Some ceremonial head of state posts intentionally go unfilled, such as in the case of the Empty Throne of Andor. A historical head of state on Earth was the post of the monarch of the United Kingdom of Great Britain - which lasted into at least the 21st century.
Heads of state who double as heads of government include the posts of President of the United Federation of Planets (Nanietta Bacco), Grand Nagus of the Ferengi Alliance (Rom), and Castellan of the Cardassian Union (Alon Ghemor). Heads of state of Federation Members include the posts of President of the Trill Union, First Minister of the Republic of Bajor, President of United Earth, President of the United States of America (subdivision of United Earth), and Governor of Cestus III.