Let's say you give a presentation where you intent to talk about A, B and C. But before you get to talk about B you feel that since it's a subject in a field not everyone in the audience might be familiar with (or remember well) you have to give a short introduction/overview into the field first before really talking about the subject. In some cases it might even seem unexpected to the audience members which is why you might want to explicitly preface this part of your presentation.
Now how would you call this kind of interruption?
- A digression?
- A tangent?
- An excursus? (Which is what you might call it in German.)
(While a single word would be nice a phrase would work as well.)