An account used by the United States federal government to record excess contributions paid into the Social Security system. The Social Security Trust Fund is used when contributions made by workers and employers exceed the amount currently needed to fund the Social Security system and those retired workers currently collected Social Security benefits. The monies held within the fund are invested in interest-bearing federal securities in order to increase while being held in trust. The Social Security Trust Fund was originally created to account for an anticipated future shortfall in benefits needed to pay out retirees. Following an increase in the Social Security payroll tax in the 1980s, the excess contributions from the tax increase were deposited into the Social Security Trust Fund to be used at a future date when the current assets of the Social Security system are no longer sufficient to cover their obligation.