Final answer:
A government ruled by one person is called a monarchy, which is different from an oligarchy, where a small group rules, and a democracy or republic, where the people or their representatives have power.
Step-by-step explanation:
The ancient Greeks conceived three fundamental forms of government. The form of government ruled by one individual is called a monarchy. In a monarchy, a single person, known as a monarch, rules until they die or abdicate the throne. This concept contrasts with an oligarchy, where a small group of elite people holds political power, and a democracy, where power is given to the majority of citizens.
In the United States, after the war for independence, revolutionaries turned to republicanism, a form of government where there is no hereditary ruling class and representatives chosen by the people lead the nation. A republic stands in contrast to a monarchy and relies on the principle of popular sovereignty.
Overall, each form of government has distinct characteristics. A monarchy has a sole ruler, an oligarchy is ruled by an elite few, and a democracy and republic seek to represent the will of the people, with the republic doing so through elected representatives.