The second is the duration of 9 192 631 770 [[Period (physics)  of the radiation corresponding to the transition between the two Hyperfine structure  of the ground state  of the caesium-133  atom .
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.
- 60 seconds = 1 minute 
- 3 600 seconds = 1 hour 
- 86.4 kiloseconds (86 400 seconds) = 1 day  (in the SI sense)
See also explanation at 
The definition of the second was later refined at the 1997 meeting of the Bureau International des Poids et Mesures  to include the statement
- This definition refers to a caesium atom at rest at a temperature of 0 K.
- Official BIPM definition of the second
- Public domain article about definition of seconds and leap seconds
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