Expression/word for talking about a seemingly different subject for a while?

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.)

People who veer off topic can be said to digress, ramble or drift, all of which have different connotations.

In this case, the individual knows what he or she wants to talk about, so they presumably aren't rambling or drifting. It would be better described as a digression.

But if the digression occurs at the very beginning of the talk, then I think it would be more accurate to call it something else, like an introduction or preface.

More possibilities in the spirit of introduction and preface:

background    "Let me provide some background."
groundwork   "Let's start with some groundwork."
lemma     "Lemma A will be used in the proof of Theorem T." (mathematics)