Absolutism (AKA "autocracy) was a legal and political philosophy used to justify nearly-unlimited power held by reigning monarchs (kings and queens). Its roots in European history come from the divine right of kights theory, a medieval notion that held that the monarch's power was virtually unlimited, because he/she received his/her right to rule from God himself.
Absolutism came into prominence in early modern Europe (c. 1400-1700) and is exemplified in such famous monarchs as Elizabeth I Tudor of England, Henry VIII of England, Catherine the Great of Russia, Charles V of the Holy Roman Empire, and Philip II of Spain. Though it is often used in a European context, the term could be aptly used to describe rulers from other global regions from the early modern period, such as Akbar the Great of Mughal India or Suleiman the Magnificent, Ottoman Emperor.
Note: The Age of Absolutism faced a critical challenge with the coming of the Enlightenment (c. 1650-1800), which challenged the notion of accepting unlimited monarchical power and opted instead for a social contract between the government and the people.