- 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.
Origin of zero hour
First recorded in 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
- 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