An atomic clock was a clock which used atomic vibrations to measure time.
Professor Wagg designed an atomic clock for the countdown to the year 2000 and the new millennium. However, the Eighth Doctor had to steal the clock's beryllium chip, rendering it inoperable. (TV: Doctor Who)
Bill Potts, with the aide of the Twelfth Doctor as a tutor, wrote an essay with the elongated title of "Laser cooling of ions: atomic clocks and quantum jumps." (TV: The Pilot)