The amount between the maximum liability and the sum of the fixed costs and paid claims that some employers choose to not fund.
The program is expected to have $3.7 trillion in unfunded liabilities over the next 75 years. Over the course of 75 years the Social Security Administration will not be able to pay $3.7 trillion of its currently scheduled obligations.
Generally, any amount by which the assets of a pension plan are less than its liabilities.