Despotism is government by a singular authority - either a single person or tightly knit group - which rules with absolute power. The word implies tyrannical rule; it suggests a form of government which exercises exacting and near-absolute control over all of its citizens.
A variation of the term is benevolent despotism, which refers specifically to a form of rulership that came to prominence in the eighteenth century. In this instance, the absolute monarchs ruling certain nations used their authority to institute a number of reforms in the political and social structures of their countries. This movement was probably largely triggered by the ideals of the Enlightenment.