48 C.F.R. § 39.002
As used in this part—
Modular contracting means use of one or more contracts to acquire information technology systems in successive, interoperable increments.
National security system means any telecommunications or information system operated by the United States Government, the function, operation, or use of which—
[61 FR 41470, Aug. 8, 1996, as amended at 62 FR 274, Jan. 2, 1997; 62 FR 44830, Aug. 22, 1997; 63 FR 9068, Feb. 23, 1998; 66 FR 2133, Jan. 10, 2001; 79 FR 70344, Nov. 25, 2014]