It can be lots of things, depending on the question. In the viva an answer could be…
- …an explanation you’re presenting as fact. You believe with certainty that what you’re saying is “true”.
- …an opinion. You have an idea, some reasons and an argument that feels plausible to you.
- …a simple fact. Information pulled from your memory.
- …a guess. An idea plus hope.
Questions in the viva could lead to all of these kinds of answers, and more. For a lot of questions you might face, the only answer you can give would be an opinion. You may face a question which has no definite answer, and your examiners know that too. They want to see how you think about your subject.
For all the questions you face, best to be honest with yourself (and your examiners) about which kind of answer you’re giving.