A monarchy is a form of government in which sovereignty is actually or nominally embodied in one individual reigning until death or abdication. They are called the monarchs. Forms of monarchy differ widely based on the level of legal autonomy the monarch holds in governance, the method of selection of the monarch, and any predetermined limits on the length of their tenure. When the monarch has no or few legal restraints in state and political matters, it is called an absolute monarchy and is a form of autocracy. Cases in which the monarch's discretion is formally limited are called constitutional monarchies. In hereditary monarchies, the office is passed through inheritance within a family group, whereas elective monarchies use some system of voting. Each of these has variations: in some elected monarchies only those of certain pedigrees are eligible, whereas many hereditary monarchies impose requirements regarding the religion, age, gender, mental capacity, and other factors. Occasionally this might create a situation of rival claimants whose legitimacy is subject to effective election. Finally, there have been cases where the term of a monarch’s reign is either fixed in years or continues until certain goals are achieved: an invasion being repulsed, for instance. Thus there are widely divergent structures and traditions defining monarchy.