master clock

master clock (plural master clocks)

1. A precision clock that provides timing signals to synchronise slave clocks as part of a clock network.
   • 1960 September, M. J. Wilson, “Harlow Town station opened”, in Trains Illustrated, page 542:

     Special details of the new station are the crystal-regulated master clock in the concourse, which controls 11 "slave" clocks and will also control those to be installed in the Harlow Mill goods depot; [...].

• slave clock