H-hour

American  

[eych-ou^uhr, -ou-er] / ˈeɪtʃˌaʊər, -ˌaʊ ər /

noun

Military.

1. the time, usually unspecified, set for the beginning of a planned attack.

---

H-hour British  

noun

1. Also called: zero hour.  military the specific hour at which any operation commences

---

Etymology

Origin of H-hour

First recorded in 1918; H (for hour ) + hour; cf. D-day

Example Sentences

Stormed Utah Beach at H-hour on D-Day as part of the 4th Infantry Division.

From Washington Post

Those 2,000 ships had set out from different ports, at different speeds, on different courses; each had its appointed station for H-hour.

From Time Magazine Archive

With room in its hangar for 30 troop-carrying helicopters, the ship is designed to put two reinforced Marine rifle companies ashore by H-hour plus three minutes.

From Time Magazine Archive

So complete was Big Three solidarity that a General Motors public relations man was overheard phoning his Chrysler counterpart to tell him that H-hour for the release of the statements had come.

From Time Magazine Archive

We breathed the standard helium-oxygen mix at normal pressure until about four hours before H-hour.

From The Trouble with Telstar by Schoenherr, John