zero hour
Preserved but no longer current.
A military term (first used in World War I) for the exact time at which an attack etc., is to be begun. From this are timed the consequent operations, e.g. zero + 3 means 3 minutes after zero hour. Succeeded in World War II by H-hour.
Era: ca. 1898 (Victorian)
Sources & notes
Reproduced from Brewer’s Dictionary of Phrase and Fable (1898 edition, public domain; archive.org, retrieved 2026-07-31; OCR text, minor errors possible). Reflects the scholarship and attitudes of its era.
Common questions
What does “zero hour” mean?
A military term (first used in World War I) for the exact time at which an attack etc., is to be begun. From this are timed the consequent operations, e.g. zero + 3 means 3 minutes after zero hour. Succeeded in World War II by H-hour.
Is “zero hour” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.