An ideology is a set of beliefs or philosophies attributed to a person or group of persons, especially those held for reasons that are not purely epistemic, in which "practical elements are as prominent as theoretical ones." Formerly applied primarily to economic, political, or religious theories and policies, in a tradition going back to Karl Marx and Friedrich Engels, more recent use the term as mainly condemnatory.

The term was coined by Antoine Destutt de Tracy, a French Enlightenment aristocrat and philosopher, who conceived it in 1796 as the "science of ideas" to develop a rational system of ideas to oppose the irrational impulses of the mob. In political science, the term is used in a descriptive sense to refer to political belief systems.