Second (unit of time)

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 is derived from the division of the day first into 24 hours, then into 60 minutes, and finally into 60 seconds (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, 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.
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