Definition:Time/Unit/Second

Definition
The second is the SI base unit of time.

It was initially defined as being $\dfrac 1 {60 \times 60 \times 24}$ 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.

Symbol
The symbol for the second is $\mathrm s$.