split second

noun
1.
a fraction of a second.
2.
an infinitesimal amount of time; instant; twinkling.

Origin:
1880–85

split-second, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
split second
 
n
1.  an extremely small period of time; instant
 
adj
2.  made or arrived at in an infinitely short time: a split-second decision
3.  depending upon minute precision: split-second timing

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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American Heritage
Idioms & Phrases

split second

An instant, a fraction of a second, as in Our best swimmer came in a split second before theirs. This expression alludes to a stop watch that has two second hands, one above the other, for timing more than one athlete or intervals of a race by a single athlete. Each hand can be stopped independently of the other, so a second can be "split" when one second hand stops a fraction of a second after the other. [c. 1880]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
He can have had only a split second to realise what was about to happen.
When the fingers slipped up, they paused a split second longer than usual
  before typing the next letter.
They'll also experience a split second of weightlessness as they nosedive down
  the mountainous drop.
Officiating is a difficult, thankless job in which calls must be made
  instinctually in a split second.
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