Money & budgets
trust fund
A federal accounting mechanism dedicating specific revenues to specific programs, such as Social Security, Medicare Part A, and the Highway Trust Fund.
In full
A federal trust fund is a bookkeeping device, not a vault: incoming payroll or excise taxes are credited to it and lent to the Treasury in exchange for special securities. When a fund's projected balance runs out, the law generally limits spending to incoming revenue, which is what trust fund exhaustion dates actually mean.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.