Definition:Time/Unit/Second

Definition
The second is the SI unit of time.

It was initially defined as being $$\frac 1 {60 \times 60 \times 24}$$ the length of a day.

Since 1967, the second has been defined to be
 * the duration of $$9 \, 192 \, 631 \, 770$$ periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.