[DuncanWaring]

1. Time required for light to travel 299,792,458 meters.

Alternatively:

2. 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.

[dirtboy]

2. 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.

At what temperature?