zero hour

See more synonyms for zero hour on
  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 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for zero hour

zero hour

  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