Define second…
The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium – 133 atom.
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The second is the base unit of time in the International System of Units, commonly understood and historically defined as \u200b¹⁄₈₆₄₀₀ 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.