An unfunded liability of a government is a debt to a fund designed to make payments to people or entities, meaning that there's no money available to pay the debt obligations. These debt obligations are expected to be paid when the government receives its income (taxes) or by investing the funds in different investment vehicles.
Unfunded liabilities in the United States are the debt to the Medicare fund (parts A, B and D), Social Security Fund, federal employee and veterans benefits. As of now, the sum of all unfunded liabilities is 172.7 trillion US$
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