Definition
The second 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. This definition refers to a caesium atom at rest at a temperature of 0 K.
It approximates the beat or half period (one swing, not back and forth) of a pendulum one metre in length.
Origin
Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the minute we know today.
Conversions
60 seconds = 1 minute
3 600 seconds = 1 hour
86.4 kiloseconds (86 400 seconds) = 1 day (in the SI sense) ...International System of Units
And if you've got a few extra seconds
... this is also very interesting! 
Explanation
The factor of 60 comes from the Babylonians who used factors of 60 in their counting system. The hour had previously been defined by the ancient Egyptians in terms of the rotation of the Earth (indirectly, of course, as they were unaware that the earth rotated) as 1/24 of a solar day. This made the second 1/86,400 of a solar day. With the development of clocks keeping mean time (as opposed to the apparent time displayed by sundials), the second became 1/86,400 of a mean solar day.
In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made during the eighteenth and nineteenth centuries. The second thus defined is
the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. Reference to the year 1900 does not mean that this is the epoch of a mean solar day of 86,400 seconds. Rather, it is the epoch of the mean tropical year of 31,556,925.9747 seconds of ephemeris time. Ephemeris Time (ET) was defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion. The length of the second was based on 1/86,400 of the mean solar day between the years 1750 and 1892.
With the development of the atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the rotation of the earth.
Following several years of work, two astronomers at the United States Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second. Using a common-view measurement method based on the received signals from radio station WWV, they determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units (SI) 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.
The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second.
The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement
This definition refers to a caesium atom at rest at a temperature of 0 K.
This refinement limits the definition to only the central frequency of the broad spectrum of frequencies emitted by a typical atomic clock, which contains a caesium vapor containing many atoms moving rapidly in all directions at once. The resulting Doppler effect broadens the atoms' basic frequency so that it covers a broad spectrum of frequencies. Furthermore, it indicates that the ultimate atomic clock would contain a single caesium atom at rest emitting a single frequency.
So there you go ... who'd have thought!!! And next time you tell someone to 'wait a second', do you really mean it?
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