zero hour

- the exact time when an attack or other military action will start

The air force planes waited until zero hour in order to start out on their bombing mission.

zero hour

- the time when an important decision or change is supposed to come

We waited for zero hour and the time when the new computer system was supposed to begin operating.

zero in on

- adjust a gun so that it will hit a target, aim at

The soldiers zeroed in on the target and began to fire their guns.

zero in on

- give one`s full attention to

We spent the morning zeroing in on the problem of what to do with the excess space in our office.

zonk out

- fall asleep very quickly

As soon as I got home last night I immediately zonked out.