5 C.F.R. § 880.103
For purposes of this part-
Annuitant means an individual who has separated from the Federal service with, and has retained, title to a CSRS or FERS annuity, has satisfied the age and service requirements for commencement of that annuity, and has filed an application for that annuity;
Associate Director means OPM's Associate Director for Retirement and Insurance or his or her designee;
Authorized institution means a government organization or official legally charged with making determinations of death in the State or country of the missing annuitant's domicile, citizenship, or disappearance;
CSRS means the Civil Service Retirement System established in subchapter III of chapter 83 of title 5, United States Code;
FEGLI means the Federal Employees Group Life Insurance program established in chapter 87 of title 5, United States Code;
FEHBP means the Federal Employees Health Benefits Program established in chapter 89 of title 5, United States Code;
FERS means the basic benefit portion of the Federal Employees Retirement System established in subchapters I, II, IV, V, and VI of chapter 84 of title 5, United States Code; FERS does not include benefits under the Thrift Savings Plan established under subchapters III and VII of chapter 84 of title 5, United States Code;
Missing annuitant means an individual who has acquired the status of missing annuitant under § 880.203(b) .
5 C.F.R. §880.103