Second

The second is the base unit of time in the International System of Units, and the one every other base unit's modern definition is now built on top of. It is defined as the duration of 9,192,631,770 cycles of the radiation given off by a caesium-133 atom switching between two specific energy states, a fixed atomic property that a caesium clock can reproduce anywhere with extreme precision, far steadier than any earlier standard based on the length of a day or a year. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/