tangent

a. 接触的,切线的,相切的
n. 切线,正切

The teacher was telling us about the poet's life, when he went off at a tangent and started talking about his war experiences.

老师在给我们讲述诗人生平故事的时候，突然把话题一转，讲开他自己的战争经历来了。

It's not easy to follow her think because she's always go off at a tangent.

要跟上她的思路并非易事，因为她老是突然扯到题外去。