zero hour

American  

noun

1. the time set for the beginning of a military attack or operation.

2. the time set for the beginning of any event or action.

3. a decisive or critical time.

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zero hour British  

noun

1. military the time set for the start of an attack or the initial stage of an operation

2. informal a critical time, esp at the commencement of an action

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Etymology

Origin of zero hour

First recorded in 1915–20

Example Sentences

The sun was only seen for 30 minutes, putting it on track to be the darkest December since 1934 when there was so little light meteorologists "rounded it down to zero hours".

From Barron's

Sonia Bravo, 40, who hawks Christmas trinkets from a makeshift stand, has also heard that “zero hour” is imminent.

From Los Angeles Times

The structure of the jobs market in the UK makes that challenging to replicate - for example there are more insecure zero hours contracts.

From BBC

Agency workers who choose to be on zero hour contracts will also be made eligible for compensation if their shifts are changed at short notice, it is understood.

From BBC

From Brighton, London, Swindon, and Wrexham across to Leeds there have been zero hours of sunshine recorded in the last seven days, compared to 40 hours in north-west Scotland.

From BBC