1. Type of employer’s funding instrument for retirement accounts and employee benefits. The other types are an insured plan or combination plan . 2. Separate account in the U.S. Treasury, mandated by Congress, whose assets may be used only for a specified purpose. For the supplementary medical insurance (SMI) trust fund, monies not withdrawn for current benefit payments and administrative expenses are invested in interest-bearing federal securities, as required by law; the interest earned is also deposited in the trust fund.