zero hour
noun
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
Words nearby zero hour
zero,
zero balancing,
zero defects,
zero gravity,
zero grazing,
zero hour,
zero in,
zero in on,
zero option,
zero population growth,
zero stage
British Dictionary definitions for zero hour
zero hour
noun
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