The unequal distribution of valuable goods — income, wealth, life expectancy, political power, education, recognition, health — across persons and groups within a society. Every serious political tradition has had to take a position on inequality: which kinds matter, which are tolerable, which are produced by injustice, and what (if anything) should be done about them.
The empirical study of inequality has been transformed in the last two decades. Thomas Piketty and his collaborators have produced a long-run record of income and wealth concentration in the advanced economies showing a U-shape across the 20th century: very high inequality before 1914, sharp compression through the World Wars and the welfare-state settlement, and a return to pre-1914 levels in many countries since the 1980s. The United States now leads the rich world in concentration of both income and wealth. The mechanisms — financialization, the decline of organized labor, regressive tax policy, deregulation, the rise of monopoly rents, the internationalization of capital — are well documented; the political will to reverse them, less so.
The normative arguments are older. Marx read inequality as the structural product of capitalist exploitation. Rawls's A Theory of Justice (1971) made the contemporary liberal case that inequalities are justified only insofar as they benefit the worst-off. Sen's capability approach asks not merely about distributions of resources but about what people are actually able to do and be. American writers like Anand Giridharadas and Naomi Klein focus on the political mechanisms by which extreme inequality is laundered as philanthropy or naturalized as neoliberal common sense. The Racial Capitalism tradition insists that economic inequality and racial hierarchy are jointly produced and cannot be analyzed apart.