An entitlement is a government program guaranteeing access to some benefit by members of a specific group and based on established rights or by legislation.
Congress does not pass an annual appropriation; instead, expenditure on the program automatically increases or decreases with the number of claims against eligibility criteria.
The government must provide the benefits even if it is insolvent, has reached the debt ceiling, or has not passed a budget.
If the trust fund is exhausted, this means benefits must be paid from discretionary sources such as Income Tax revenue; it does not mean benefits will run out (although it implies other programs will need to be significantly curtailed given the size of entitlement programs relative to discretionary spending).
In order to reduce the government's overall debt and interest expense, trust funds make inter-fund loans to the discretionary fund; otherwise, the government would simultaneously retain uninvested cash in reserves and borrow more at higher rates.
Into the 21st century, the meaning has been used to refer also to benefits, like those of the food stamps program, which people become eligible to receive without paying into a system.