Second
Symbol: s
The second is the base unit of time in the International System of Units (SI).
The second has been defined to be 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 133Cs atom.
Until 1956 the second thus defined was the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.