It's a word professors like to use when they talk about a topic and spontaneously decide to discuss a related topic before coming back to the main topic. I cannot seem to remember the word for that though. Detour is not the word I'm thinking of, but it's something in the same vein.
1 Answer
Digression fits the circumstances you describe. It is the act of leaving the main subject you are writing or talking about, then writing or talking about something else before returning to the first theme. Dictionary definitions abound and I need not repeat them here, having now (I hope) awakened your memory.