Theocracy is a form of government in which a deity, or religious institution, is the source from which all authority derives. The Oxford English Dictionary has this definition:
1. a system of government in which priests rule in the name of God or a god.
1.1. the commonwealth of Israel from the time of Moses until the election of Saul as King.
An ecclesiocracy is a situation where the religious leaders assume a leading role in the state, but do not claim that they are instruments of divine revelation: for example, the prince-bishops of the European Middle Ages, where the bishop was also the temporal ruler. Such a state may use the administrative hierarchy of the religion for its own administration, or it may have two "arms"—administrators and clergy—but with the state administrative hierarchy subordinate to the religious hierarchy. Theocracy differs from theonomy, the latter of which is government based on divine law.
The papacy in the Papal States occupied a middle ground between theocracy and ecclesiocracy, since the Pope did not claim he was a prophet who received revelation from God and translated it into civil law.
Religiously endorsed monarchies fall between theocracy and ecclesiocracy, according to the relative strengths of the religious and political organs.
Most forms of theocracy are oligarchic in nature, involving rule of the many by the few, some of whom so anointed under claim of divine commission.