Which form of government is characterized by rule by a single individual with unlimited power?

The form of government characterized by rule by a single individual with unlimited power is autocracy. In an autocracy, the ruler holds absolute authority, and there are few, if any, checks on their power. This can manifest in various ways, such as through dictatorial control or absolute monarchy, where the monarch has unrestricted rights over the state's governance.

Monarchy does involve a single ruler, but not all monarchies are autocratic; some are constitutional, meaning the monarch's powers are limited by law or a governing body. Oligarchy refers to rule by a small group of individuals, rather than a single person, which distinctly differs from the concept of autocratic rule. A republic is characterized by elected representatives and an elected leader (usually a president), which also does not align with the definition of unlimited power under a single ruler. Thus, autocracy uniquely embodies the principle of concentrated power in a single individual unrestrained by laws or constitutional limits.