What Does It Mean?
What does this mean? What does that mean? What’s the difference between X and Y?
If there are any terms that you can remember being asked about a lot during your PhD, then take time to refresh your memory before your viva. If there are terms you use regularly while you do your work, be sure that they mean what you think they mean. Be sure that there aren’t edge cases or extra points you’ve forgotten.
You don’t need to know every fact, detail, reference and idea, but take time to unpick what common terms mean, especially if you’ve used them a lot.