The Second
The second is the base unit of time in the International System of Units.
Currently in the International System of Units the second is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s−1.
An example of how seconds are measured. Don't watch too long. Once they're gone they don't come back.