The term political radicalism (or simply, in political science, radicalism) denotes political principles focused on altering social structures through revolutionary means and changing value systems in fundamental ways. Derived from the Latin radix (root), the denotation of radical has changed since its eighteenth-century coinage to comprehend the entire political spectrum—yet retains the "change at the root" connotation fundamental to revolutionary societal change. Historically, radicalism has referred exclusively to the radical left (under the single category of far-left politics) and rarely incorporating far-right politics, though these may have revolutionary elements; the prominent exception is in the United States where some consider radicalism to include both political extremes of the radical left and the radical right. In traditional labels of the spectrum of political thought, the opposite of radical on the "right" of the political spectrum is termed reactionary.
The nineteenth-century Cyclopaedia of Political Science (1881, 1889) reports that "radicalism is characterized less by its principles than by the manner of their application". Conservatives often used the term radical pejoratively, whereas contemporary left radicals used the term conservative derogatorily; thus contemporary denotations of radical, radicalism, and political radicalism comprehend far left (hard left, radical left), and far right (hard right, radical right).
The Encyclopædia Britannica records the first political usage of radical as ascribed to the British Whig Party parliamentarian Charles James Fox, who, in 1797, proposed a "radical reform" of the electoral system, franchise to provide universal manhood suffrage, thereby, idiomatically establishing radical to denote supporters of the reformation of the British Parliament. Throughout the nineteenth century, the term was combined with political notions and doctrines, thus working class radicalism, middle class-, philosophic-, democratic- bourgeois-, Tory-, and plebeian radicalism. In the event, politically influential radical leaders give rise to their own trend of political radicalism, e.g. Spencean radicalism and Carlilean radicalism. Philosophically, the French political scientist Jean-Jacques Rousseau (1712–78), is the principal theoretician proposing political radicalism as feasible in republican political philosophy, viz the French Revolution (1789–99), and other modern revolutions—the antithesis to the liberalism of John Locke.