Second

The second is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. The current and formal definition in the International System of Units (SI) is more precise:The second [...] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, 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. Read more on Wikipedia