President is a common title for the head of state in most republics. Depending on the country, a president could be head of government, a ceremonial figurehead, or something between these two extremes.

The functions exercised by a president vary according to the form of government. In parliamentary republics, they are usually, but not always, limited to those of the head of state and are thus largely ceremonial. In presidential and selected parliamentary (e.g. Botswana and South Africa) republics the role of the president is more prominent, encompassing the functions of the head of government. In semi-presidential republics, the president has some discretionary powers like over foreign affairs, appointment of the head of government and defence, but they are not themselves head of government. A leader of a one-party state may also hold the position of president for ceremonial purposes or to maintain an official state position.

The title "Mr. President" may apply to a person holding the title of president or presiding over certain other governmental bodies. "Mr. President" has subsequently been used by governments to refer to their heads of state. It is the conventional translation of non-English titles such as Monsieur le Président for the president of the French Republic. It also has a long history of usage as the title of the presiding officers of legislative and judicial bodies. The speaker of the House of Commons of Canada is addressed as président de la Chambre des communes in French and as Mr. Speaker in English.

The title president is derived from the Latin prae- "before" + sedere "to sit". The word "presidents" is also used in the King James Bible at Daniel 6:2 to translate the Aramaic term סָרְכִ֣ין (sā·rə·ḵîn), a word of likely Persian origin, meaning "officials", "commissioners", "overseers" or "chiefs". As such, it originally designated the officer who presides over or "sits before" a gathering and ensures that debate is conducted according to the rules of order (see also chairman and speaker), but today it most commonly refers to an executive official in any social organization. Early examples are from the universities of Oxford and Cambridge (from 1464) and the founding president of the Royal Society, William Brouncker, in 1660. This usage survives today in the title of such offices as "President of the Board of Trade" and "Lord President of the Council" in the United Kingdom, as well as "President of the Senate" in the United States (one of the roles constitutionally assigned to the vice president). The officiating priest at certain Anglican religious services, too, is sometimes called the "president" in this sense.

The most common modern usage is as the title of a head of state in a republic. The first usage of the word president to denote the highest official in a government was during the Commonwealth of England.

Thomas Hungerford, first recorded speaker of the English House of Commons in the rolls of the Parliament of England in 1376/7,, used the title, "Mr. Speaker", a precedent followed by subsequent speakers of the House of Commons.

After the abolition of the monarchy, the English Council of State, whose members were elected by the House of Commons, became the executive government of the Commonwealth. The Council of State was the successor of the Privy Council, which had previously been headed by the lord president; its successor the Council of State was also headed by a lord president, the first of which was John Bradshaw. However, the lord president alone was not head of state, because that office was vested in the council as a whole.

The speaker of the House of Commons of Canada, established in 1867, is also addressed as "Monsieur le Président" or "Madame la Présidente" in French.