The basic unit of time corresponding to 1/86,400 of the mean solar day, which is the period of rotation about its own axis of an ideal Earth describing a circle round the Sun in one year, at a constant speed and in the plane of the Equator.
In the post-Second World War period, atomic clocks reached a degree of precision that could demonstrate the infinitesimal irregularities (a few hundredths of a second per year) of the Earth's rotation about its own axis. It was then decided to redefine the reference standard. This new definition was ratified by the 13th General Conference on Weights and Measures in 1967 as "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."
Conventionally, the second is subdivided into tenths, hundredths, thousandths (millisecond), millionths (microsecond), billionths (nanosecond) and trillionths (picosecond).
In a watch, seconds are indicated by the fourth wheel and the seconds hand.