The title of president goes back to Latin praesident-, praesidens, meaning "ruler," which, in turn, derives from the present participle of praesidēre, "to guard" or "to preside over." Today, president commonly refers to the chief officer of an organization, such as a corporation or educational institution, who is entrusted with the direction and administration of its policies. In the rarefied world of politics, the title designates the elected official occupying the position of chief of state in a republic.
In countries governed by a parliament, the president has only minimal political power—that's not the case in the U.S., where the president is not only the chief of state but is its chief political executive. As chief executive, he or she must ensure that laws of the land are faithfully executed, which is done through various executive agencies and with the aid of the presidential cabinet; in addition, the president is vested with the power to sign a bill into law or veto one passed by Congress. If that's not enough, the president, when called upon, dons the hat of commander in chief of the armed forces.
Prior to the creation of the office of the presidency of the United States (with the ratification of the U.S. Constitution in 1788), being president wasn't so demanding. In Colonial America, the title president was bestowed on the chief magistrate of some British colonies who were elected by a council. That title was then transferred to the head of the early state governments (as in "President of the Pennsylvanian State"); president in this sense was replaced in all states by governor by the start of the 19th century. Before George Washington, the "President of the United States" was a presiding officer over the Continental Congress and of the Congress established by the Articles of Confederation.
On Monday last the Hon. Elias Boudinot, Esq; was elected President of the United States in Congress assembled.
— The Pennsylvania Gazette, 6 Nov. 1782