In spacecraft, a Command-Loss Timer (CLT) is a software timer within the command and data system (CDS) which is restarted every time the spacecraft receives a command from Earth.
[1] If the CLT times out, it is assumed that the spacecraft's receiver has failed to reliably receive messages and, in response, the CDS will attempt to restore communication[1] or perform safing procedures, or both.
A Command Loss Timer is effectively a software watchdog timer which, upon timeout, initiates corrective actions that will in some cases swap to redundant communication hardware.
[1] Robotic spacecraft are accessible to human operators only by command telemetry.
Consequently, a spacecraft need to be self-reliant, because its embedded systems cannot be physically accessed if a communication fault occurs.