The second is 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.

• 秒是铯-133原子基态的两个超精细能级之间跃迁所对应的辐射的9192631770个周期的持续时间。[根据第13届国际计量大会（1967），决议1