A second is the period of a pendulum with a length of one meter. :-)
(But not really; that was originally considered as a way to standardize the meter, IIRC, but the period of a pendulum varies too much even over the area of France to be used as a concrete standard.
But the relationship is remarkably close for a coincidence, like the way a rod is almost exactly five meters.)
The calculation of a second has been updated in 2019
The second is defined by taking the fixed numerical value of the cesium
frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of
the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which
is equal to s−1.
The conference where this was made official was quite a beautiful, upbeat demonstration of people from around the world coming together to agree upon a change:
(But not really; that was originally considered as a way to standardize the meter, IIRC, but the period of a pendulum varies too much even over the area of France to be used as a concrete standard. But the relationship is remarkably close for a coincidence, like the way a rod is almost exactly five meters.)