A second (symbol s) is the base unit of time in the International System of Units (SI Units).

In 1967 it was defined as the duration of 9,192,631,770 'oscillations' of a caesium 133 atom (^{133}Cs).

The full definition now refers to a caesium atom at rest and at a temperature of absolute zero (0 Kelvin) and 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"

To allow for the relativistic affects of gravitational time dilation at different altitudes, the second measured for any atomic clock is also corrected to mean sea level.

Prior to the development of atomic clocks, the second had been defined as a fraction (1/86,000) of a 'mean solar day', the length of which was based on astronomical measurements.

The minute, hour and day are derived units:

- 1 minute = 60 s
- 1 hour = 60 × 60 = 3,600 s
- 1 day = 24 × 60 x 60 = 86,400 s

*(Note that the correct abbreviation for second is***s**,*not***sec***.)*