WordReference Random House Unabridged Dictionary of American English © 2026
ze′ro hour′, 
    1. Militarythe time set for the beginning of a military attack or operation.
    2. the time set for the beginning of any event or action.
    3. a decisive or critical time.
    • 1915–20

Collins Concise English Dictionary © HarperCollins Publishers::
zero hour n
  1. the time set for the start of an attack or the initial stage of an operation
  2. informal a critical time, esp at the commencement of an action
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'zero hour' also found in these entries (note: many are not synonyms or translations):
H-hour

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