Second (physics)
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The second is a unit of time, currently defined in the 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 cesium 133 atom. Previously, the second had been defined as 1/86 400 of a day, with 60 seconds making one minute, 60 minutes making one hour, and 24 hours making one day, and the day being defined as one mean solar day using astronomical observations. The SI second is 1/86 400 of a mean solar day as measured in 1820; since then, the rotation of the earth has slowed, and the mean solar day is approximately 86 400.002 seconds long.
The word "second" is often used colloquially to mean any very short amount of time.