Question Hour:
Definition: The first hour of every parliamentary sitting is slotted for the Question Hour. However, in 2014 the Question Hour was shifted in the Rajya Sabha from 11 am to 12 noon.
During this one hour, Members of Parliament (MPs) ask questions to ministers and hold them accountable for the functioning of their ministries.
The questions can also be asked to the private members (MPs who are not ministers).
Zero Hour:
Zero Hour is an Indian parliamentary innovation. It is not mentioned in the parliamentary rules book.
Under this, MPs can raise matters without any prior notice.
The zero hour starts immediately after the question hour and lasts until the agenda for the day (i.e. regular business of the House) is taken up.
In other words, the time gap between the question hour and the agenda is known as zero hour.