An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on the fact that atoms have quantised energy levels, and transitions between such levels are driven by very specific frequencies of electromagnetic radiation. This phenomenon serves as the basis for the SI definition of the second:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ${\displaystyle \Delta \nu _{\text{Cs}}}$, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹.

This definition underpins the system of TAI, which is maintained by an ensemble of atomic clocks around the world. The system of UTC — the basis of civil time — implements leap seconds to allow clock time to stay within one second of Earth's rotation.

The accurate time-keeping capabilities of atomic clocks are also used for navigation by satellite networks such as the EU’s Galileo Programme and the United States’ GPS. The timing accuracy of the atomic clocks matters because even a timing error of 1 nanosecond (10⁻⁹ s) corresponds to a positional error of roughly 30 cm when multiplied by the speed of light.

The main variety of atomic clock in use today employs caesium atoms (or ions) cooled to near absolute zero. For example, the United States’ primary standard, the NIST caesium fountain clock named NIST-F2, operates with a relative uncertainty around 10⁻¹⁶.