A single, hereditary ruler or head of state. May be styled king, prince, (arch-)duke, queen, princess, (arch-)duchess, shah, sultan, czar, emperor, empress, pharaoh, or such titles. A state ruled by a monarch is called a monarchy.
Few monarchs today exercise absolute authority. Although there are a number of hereditary monarchies still existing in the world, many countries with hereditary royalty are de facto ruled by a democratically elected leader such as a prime minister, while the monarchy continues to hold a symbolic or ceremonial position (eg. United Kingdom; see also constitutional monarchy).
The system for succession to the throne varies from monarchy to monarchy. Traditionally, succession to the eldest son of the monarch has been most common; if the monarch had no sons, the throne would pass either to the eldest daughter, or to the nearest male relative, depending on whether the monarchy accepts female ruler. Some monarchies have abolished this preference for males, and the eldest child of the monarch ascends to the throne, be that child male or female, e.g. some European monarchies. In some monarchies, e.g. Saudi Arabia, succession to the throne has passed to the monarch's next eldest brother, and only to the monarch's children after that. In some other monarchies, the monarch chooses who will be his successor, who need not necessarily be his eldest son, e.g. Jordan.