a detailed discussion or explanation of a subject, which is separate from the main subject that is being written or talked about

Before discussing this a short historical excursus will be necessary to explain how it all came about. 在讨论这个问题之前，我们有必要简短地补充一下历史背景，解释一下这一切是如何发生的。

The case histories include an 11-page excursus on Latin. 案例史包括11页有关拉丁文的附录。