In a defined benefit plan the benefit is defined regardless of the return on investment of any contributions to the plan or the period of time over which benefits are payed. In a defined contribution plan the contribution is fixed, but the benefit is dependent on the performance of the underlying investment over the period that benefits are payed. CSRS is a defined benefit plan. The FERS has a little of both defined benefit and defined contribution. Social Security is the base of the defined benefit portion, and the thrift savings plan is the defined contribution part. Participation in the thrift savings portion is voluntary, but necessary to get a retirement benefit close to what the CSRS gives.