You have to define what it means for you.
Having a good conversation?
Passing with a certain outcome?
Feeling happy on the day?
Not feeling nervous?
Remember that your definition of viva success can be a motivation to you, so it’s helpful to have something in mind.
Remember also that not every aspect of the viva is within your control. If your definition of success is not something you can directly influence it might be worth rethinking how you frame it.
What does viva success mean to you? And how does that help you work towards a good outcome?