The social relationship in which one party owes something to another — money, labor, goods, or obligation — and the institution through which that relationship has organized power, enforced hierarchy, funded states, and structured the experience of most people who have ever lived.
Money exists to measure debt, not the other way around. Before coins, before banks, there were ledgers — records of who owed what to whom. The Mesopotamian credit system is older than coinage by at least a thousand years. Debt is not a secondary institution built on top of a market economy. It is what makes market economies possible. And debt is not just economic: it is the medium through which slavery was enforced, through which empires were built, and through which ordinary people have been controlled by powerful institutions for five thousand years.