Hegemony — from the Greek hegemon, leader — is the condition in which a single state's preferences become the international system's defaults. The hegemon's currency is the reserve currency. Its language is the trading lingua franca. Its legal templates structure international agreements. Its universities train the global elite. Its values get coded into the rules. Britain in 1860, when the pound financed world trade and the Royal Navy policed the sea lanes. The United States in 1990, when the dollar, the dollar-denominated oil market, and a network of alliances spanned the globe. The hegemon does not have to be especially loved; it has to be unavoidable. The job is mostly infrastructural — running the plumbing through which everyone else's transactions must pass.
The theoretical literature on hegemony breaks roughly into three positions. Hegemonic stability theory (Charles Kindleberger, Robert Gilpin) argues that the international economy needs somebody to provide public goods — open markets, a stable currency, a security umbrella, a lender of last resort — and that those goods are systematically underprovided in the absence of a hegemon; Kindleberger blamed the depth of the 1930s Depression on Britain's inability and America's unwillingness to play that role, the interwar gap between an exhausted old leader and a reluctant new one. The Gramscian version (Robert Cox, world-systems theorists) argues that hegemony is constituted by consent as much as coercion — the hegemon governs through institutions, ideologies, and elite networks that produce voluntary alignment, so that its rule looks less like domination than like common sense. The neorealist version (Mearsheimer) is more skeptical: hegemony is unstable because rising powers will balance against the hegemon, and no state can reach global hegemony — only regional hegemony, with the oceans as a barrier that even the strongest power cannot project decisive force across. The post-1991 American moment is the richest case study — three decades in which a single state had unprecedented military dominance, the dominant currency, the leading universities, the dominant cultural exports, and the deepest financial markets, and was nevertheless unable to translate this into stable global governance on the issues it most cared about (climate, nuclear proliferation, terrorism, China's rise) — a primacy in resources that did not convert into control over outcomes, which is the central puzzle each of the three schools tries to explain.
The post-American question is whether the next system has no hegemon (a multipolar world with severe coordination problems on climate, finance, and pandemics), a contested hegemony between the US and China fought largely through technology, currency, and standards, or a regional-hegemonic order in which different powers run different geographic spheres. The first signs — competing technology stacks, attempts to trade outside the dollar, rival development banks — point toward fragmentation rather than a clean succession. None of these has been the system's default since 1945, and which one emerges will shape the macro-trajectory of the century.