A monarchy is a form of government in which a group, usually a family called the royalty, embodies the country's national identity and one of its members, called the monarch, exercises a role of sovereignty. The actual power of the monarch may vary from purely symbolical (crowned republic) to partial and restricted (constitutional monarchy) to completely despotic (absolute monarchy). Traditionally and in most cases, the monarch's post is inherited and lasts until death or abdication, but there are also elective monarchies where the monarch is elected. 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.