The term political radicalism (in political science known as radicalism) denotes political principles focused on altering social structures through revolutionary or other 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 it 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" comprise 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 producing the concepts of 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 (see Spencean radicalism and Carlilean radicalism). Philosophically, the French political scientist Jean-Jacques Rousseau (1712–1778) is the principal theoretician proposing "political radicalism" as feasible in republican political philosophy, viz the French Revolution (1789–1799) and other modern revolutions—the antithesis to the liberalism of John Locke (1632–1704).