Definition of zero hour in US English:

zero hour


  • The time at which a planned operation, typically a military one, is set to begin.

    • ‘‘Tomorrow is zero hour,’ said one of the messages.’
    • ‘Accompanied by your other three team members, you begin at zero hour of the Clone Wars.’
    • ‘As zero hour approached, Spanish-language radio stations broadcast appeals for all able-bodied Cuban-Americans to help.’
    • ‘A few minor adjustments were made to the presentation plan, and we set 3 o'clock as zero hour.’
    • ‘Nervy to begin with, he came close to panic as zero hour approached.’
    the appointed time, the appointed hour, the crucial moment, the vital moment, the critical moment, the moment of truth, the moment of decision, the point of decision, the rubicon, the critical point, the crux
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First World War: from zero + hour.


zero hour

/ˈzɪroʊ ˌaʊ(ə)r/