>The hour at which a planned military operation is scheduled to start
>The time at which a usually significant or notable event is scheduled to take place
>A time when a vital decision or decisive change must be made
>The time at which something starts
Also, in Indian Parliament, Zero Hour is an informal device available for Members of the Parliament to raise matters without any notice (which is otherwise can be raised only with 10 days advance notice). It is done between then end of question hours and beginning of the regular business in Lok Sabha. This is an Indian parliamentary innovation which is introduced in 1962 and is still continuing.
Example
>We’re at the zero hour, so someone has to make a decision
>I have so much studying left to do before the exam, and zero hour is 8 AM!