Dictionary

zero hour

noun

: the time at which an event (such as a military attack) is scheduled to begin

Full Definition of ZERO HOUR

1
a :  the hour at which a planned military operation is scheduled to start
b :  the time at which a usually significant or notable event is scheduled to take place
2
:  a time when a vital decision or decisive change must be made

Examples of ZERO HOUR

  1. <we're at the zero hour, so someone has to make a decision>

Origin of ZERO HOUR

from its being marked by the count of zero in a countdown
First Known Use: 1915

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