For most of its history, the second was defined as a fraction of the average solar day, based on the position of the sun in the sky. One second was exactly 1/86 400 of the average solar day.

In 1956, the International Committee for Weights and Measures (CIPM) proposed new definition of the second as follows:

The second is the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time

This definition was formally adopted by the General Conference of Weights and Measures (CGPM) in 1960.

In 1967, after the development of atomic clocks, a revolution in the definition of the second took place which still applies today:

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

However, on 20 May 2019, the latest redefinition of the second will come into force:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s–1.

What’s the difference? From a technical point of view – none. Microwave caesium radiation will still be used with the same defined frequency. However, the wording of the definition is to be rebuilt so that after redefinition all base SI units have consistency in construction.