zero hour

1. 1.

   the time set for the start of an action or operation

   Broaderhour

Questions about zero hour

What does zero hour mean?

zero hour means "the time set for the start of an action or operation".

Is zero hour a common word?

zero hour is banded "Uncommon" — turns up in careful writing rather than conversation. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.