President

President was a title sometimes given to heads of state, or someone 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. , 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 two of these would have been Dwight D. Eisenhower and John F. Kennedy. 

The board of trustees of the Starfleet Academy was headed by a president in the 2150s. 

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 when Spock entered pon farr. 

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 because the titles of emperor, president, first minister and seemed too "pretentious." 

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

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

Both a Human Federation president and an Efrosian Federation president were referred to 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.