zero hour
Americannoun
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the time set for the beginning of a military attack or operation.
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the time set for the beginning of any event or action.
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a decisive or critical time.
noun
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military the time set for the start of an attack or the initial stage of an operation
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informal a critical time, esp at the commencement of an action
Etymology
Origin of zero hour
First recorded in 1915–20
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
In January 2026, the clock was set at 85 seconds to midnight, the closest we’ve ever been to a designated zero hour.
From Salon ● Jun. 15, 2026
The panel has called his bluff at the zero hour, playing the other side of the same game.
From Los Angeles Times ● Oct. 13, 2022
And considering that journalists have long used the platform to document the country’s suppression of press freedom, press outlets view Twitter’s case as a sort of zero hour for Indian free expression rights.
From Slate ● Jul. 21, 2022
The report said that 20% of workers were not on permanent contracts and 11% were on zero hour contracts.
From BBC ● Jan. 27, 2022
It is at the zero hour of the big bang and the ground zero of the black hole that the mathematical equations that describe our world stop making sense.
From "Zero: The Biography of a Dangerous Idea" by Charles Seife
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
