5 C.F.R. § 1620.41
As used in this subpart:
Current contributions means contributions that must be made for the current pay date which is reported on the journal voucher that accompanies the payroll submission.
Nonpay status means an employer-approved temporary absence from duty.
Reemployed or returned to pay status means reemployed in or returned to a pay status, pursuant to 38 U.S.C. chapter 43, to a position that is subject to 5 U.S.C. 8351 or chapter 84.
Retroactive period means the period for which an employee can make up missed employee contributions and receive missed agency contributions. It begins the day after the employee separates or enters nonpay status to perform military service and ends when the employee is reemployed or returned to pay status.
Separate from civilian service means to cease employment with the Federal Government, the U.S. Postal Service, or with any other employer from a position that is deemed to be civilian Government employment for purposes of participating in the TSP, for 31 or more full calendar days.
5 C.F.R. §1620.41