Slippery term — used both as a precise intellectual-historical label and, more loosely, as an all-purpose epithet for "market-oriented policies I dislike." Taken narrowly, it refers to a distinctive 20th-century re-founding of Classical Liberalism that coalesced in the 1938 Walter Lippmann Colloquium and the 1947 Mont Pèlerin Society, developed through the work of Friedrich A. Hayek, Ludwig von Mises, Milton Friedman, and others, and reshaped actual policy from roughly the late 1970s onward.
What distinguishes neoliberalism from 19th-century classical liberalism is instructive. Early liberals tended to assume that markets were natural, emerging spontaneously once the state stopped interfering. Neoliberals — chastened by the interwar collapse — understood that markets are political constructions requiring active, ongoing state work to sustain: property rights, contract enforcement, competition law, monetary stability, and an educational and cultural order that produces citizens who think in market terms. The neoliberal state is therefore not minimal in the classical sense but reconfigured — smaller in some dimensions (nationalized industry, welfare transfers), larger in others (regulatory architecture, central banking, prisons and policing).
As a political formation, neoliberalism is usually dated from the Chilean reforms under Pinochet, the Thatcher and Reagan governments, the reshaping of international institutions (IMF, World Bank) in the 1980s, and the reforms of both centre-left parties (Blair, Clinton) in the 1990s. Its critics — Polanyian, Marxist, post-Keynesian, communitarian — blame it for rising inequality, financialization, the hollowing-out of civil society, and the populist backlash of the 2010s. Its defenders point to the expansion of global trade, the lifting of hundreds of millions out of extreme poverty in Asia, and the persistence of its assumptions even in the policy programs of its self-declared enemies.