zero hour

noun

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

Full Definition of ZERO HOUR

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

: a time when a vital decision or decisive change must be made

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from its being marked by the count of zero in a countdown

First Known Use: 1915

Synonyms: boiling point, breaking point, clutch, conjuncture, crisis, crossroad(s), crunch, crunch time, Dunkirk, exigency, extremity, flash point, head, juncture, tinderbox, emergency

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