Definition of zero hour in English:

zero hour

Syllabification: ze·ro hour

noun

The time at which a planned operation, typically a military one, is set to begin.
More example sentences
  • Accompanied by your other three team members, you begin at zero hour of the Clone Wars.
  • A few minor adjustments were made to the presentation plan, and we set 3 o'clock as zero hour.
  • As zero hour approached, Spanish-language radio stations broadcast appeals for all able-bodied Cuban-Americans to help.
Synonyms
the appointed time, the critical moment, the moment of truth, the point/moment of decision, the Rubicon, the crux
informal the crunch

Origin

World War I: from zero + hour.

Definition of zero hour in:

Get more from Oxford Dictionaries

Subscribe to remove ads and access premium resources

Word of the day dinkum
Pronunciation: ˈdiNGkəm
adjective
(of an article or person) genuine