Holdover is a clock’s ability to keep usable time after it loses its external reference, such as GPS, GNSS, or network time. The clock does not stay perfect; it drifts according to the quality of its oscillator and the holdover design, but it can remain accurate enough for operations until synchronization returns.
In cyber security, holdover matters because time underpins log correlation, certificate checks, identity systems, and event ordering in distributed services. If an attacker jams, spoofs, or blocks a timing source, a system with poor holdover may jump badly or lose trust in its timestamps. Good defenses use stable oscillators, monitored drift, authenticated time sources, and fallback paths so services can continue safely during timing loss.