President

President was the title given to the heads of state, usually of a, or the figure who presided over a governmental or corporate body.

General Santa Anna was the President of Mexico in 1836. 

The United States of America's Executive Branch was governed by a president. Trip Tucker, hearing that Mestral may have lived on Earth from 1957 until well into the 22nd century, reckoned that the Vulcan explorer would have lived through "what - thirty presidents?". The first of these would have been Dwight D. Eisenhower and John F. Kennedy. 

The head of state of the United Federation of Planets was a president. 

Altair VI elected a new planetary president in 2267. The USS Enterprise (NCC-1701), along with two other Federation starships, was scheduled to attend the president's inauguration. The Enterprise, however, diverted to Vulcan when Spock entered pon farr. 

A Starfleet court martial, such as the one James T. Kirk endured in 2267, was also overseen by a "president". 

After negotiating Cardassia's annexation into the Dominion in 2373, Dukat effectively became the emperor of the Cardassian people, though still a servant of the Founders. However, he chose to maintain his rank of Gul as opposed to Legate because he felt it was more "hands on", and Emperor, President, First Minister and Emissary seemed too "pretentious". 

Salutation
Mr. President was a formal salutation used when addressing the president of a government.

James T. Kirk referred to the Excalbian who appeared as the former President of the United States, Abraham Lincoln, as "Mr. President". 

Both the Human and Efrosian Federation presidents were referred to as as "Mr. President", as well as President Jaresh-Inyo. 

When The Doctor portrayed the President of Earth in order to conduct negotiations with a photonic lifeform, Harry Kim addressed him as "Mr. President" upon his return.