the time set for the beginning of a military attack or operation.
the time set for the beginning of any event or action.
a decisive or critical time.
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Origin of zero hour
First recorded in 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for zero hour
military the time set for the start of an attack or the initial stage of an operation
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