FERS is a defined benefit system. It replaced CSRS.
I suppose that depends on how one defines a "defined benefit system".
CSRS is a classic defined benefit system in that the employee pays a percentage of his salary into the plan and can determine what his pension will be based basically on years of service.
FERS relies heavily on a Thrift Savings Account, similiar to 401K, with employer matching some of the employee contributions.
FERS also relies heavily on Social Security, much like the private sector, as part of its plan.