Excursus is a noun that refers to a digression on a particular point.
From Latin, we get the word excurrere (EKS coo air ay) which refers to a digression. Our word of the day has undergone a number of changes, but its meaning remains the same. Don’t be put off by the formal sound of excursus. It’s a perfect word to use when talking about something written or expressed by a formal lecturer or writer.
Professor Mitchel’s history lecture was lovely except for the excursus about president Cleveland’s policies. That kind of lengthy digression on politics can often be a distraction.