Viva voce, is often translated as living voice or word of mouth. In the viva you have to answer questions and engage with your examiners. You have to demonstrate that the expertise that created your thesis is lodged in your brain.
There are other terms – thesis defence, oral exam – but I don’t think we stick with viva for tradition’s sake. A special name makes it a special thing. It’s the viva and not something else because the name makes it more important.
Calling it the viva adds something to the special status of the final exam.
You’re special too to be there.