the backward counting in fixed time units from the initiation of a project, as a rocket launching, with the moment of firing designated as zero.
the final preparations made during this period.
a period of increasing activity, tension, or anxiety, as before a deadline.

1950–55, Americanism; noun use of verb phrase count down Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
countdown (ˈkaʊntˌdaʊn)
1.  the act of counting backwards to time a critical operation exactly, such as the launching of a rocket or the detonation of explosives
2.  to count numbers backwards towards zero, esp in timing such a critical operation

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1953, Amer.Eng., in early use esp. of launches of rockets or missiles, from count (v.) + down.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Cleared with only seconds left before possibly requiring a scrub, the signal
  was finally given to continue the countdown.
Features an easel display, backlight, and countdown timer.
Preferably it should have a large red countdown timer.
Game in progress: seven turns left on the countdown clock.
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