The second (symbol: s ) is the unit of time in the International
System of Units (SI), historically defined as 1⁄86400 of a day – this factor
derived from the division of the day first into 24 hours, then to 60 minutes
and finally to 60 seconds each (24 × 60 × 60 = 86400).
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, Δ ν 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.