Head of state
A head of state is a person who represents the highest authority in a country, and who embodies that country in international relations. He or she need not hold political power in person, and someone else may be the head of government. Alternatively, the head of state may also be the head of government. A head of state may be a hereditary monarch or an elected president. A few exceptions exist (such as the Pope, who is head of state of Vatican City), but almost all heads of state in the world are either monarchs or presidents. It is also possible for a single individual to be head of state of more than one country. The exceptional example of this is Queen Elizabeth II of the United Kingdom, who is also head of state of 15 other countries that once formed the British Empire. Other anomalies are that San Marino, for example, has two heads of state, the annually elected Captains-General, and Andorra also has two heads of state, one of whom is the president of France.
The concept of heads of state is really a fairly modern idea, that cannot easily be applied to some ancient societies. For example, it might be thought that in the Roman Empire it was the emperor who was head of state, but this would be misleading. In fact, it was the consuls who retained the highest legal office, even after they lost real power, and the emperors are better thought of as heads of government. In fact, the modern concept cannot be applied in any meaningful sense.