There is no exact definition for a political revolution. However, there are two general aspects to the definition of this term.
First, it has to be a revolution. A revolution is a complete or at least extremely significant change in an entire system. A revolution is something like the Bolshevik Revolution or the French Revolution. In these revolutions, entire systems were overthrown. Systems that were monarchical and aristocratic were overthrown and people who never had power before came to have it. This is in contrast to something like the American Revolution which was much less revolutionary since the people who had power after the Revolution had generally been important and powerful before. Of course, there is no clear dividing line between a change that is truly revolutionary and one that is only minor.
Second, the revolution has to be in terms of politics. In other words, it cannot be something like the Industrial Revolution which changed the economy but not politics. It cannot be something like a social revolution that can change the way a society looks (for example) at the relationships between men and women. Instead, it has to be a major change in a society’s political system.