Definition of zero hour in English:

zero hour

noun

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

    • ‘As zero hour approached, Spanish-language radio stations broadcast appeals for all able-bodied Cuban-Americans to help.’
    • ‘Nervy to begin with, he came close to panic as zero hour approached.’
    • ‘Accompanied by your other three team members, you begin at zero hour of the Clone Wars.’
    • ‘A few minor adjustments were made to the presentation plan, and we set 3 o'clock as zero hour.’
    • ‘‘Tomorrow is zero hour,’ said one of the messages.’
    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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Origin

First World War: from zero + hour.

Pronunciation

zero hour

/ˈzɪroʊ ˌaʊ(ə)r/