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zero hour

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noun
  1. the 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.

Origin of zero hour

First recorded in 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for zero hour

zero hour

noun
  1. military 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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012