tan ‧ gent /ˈtændʒənt/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Language: Latin ; Origin: present participle of tangere ; ⇨ ↑ tangible ]
1 . go off at a tangent British English , go off on a tangent American English informal to suddenly start thinking or talking about a subject that is only slightly related, or not related at all, to the original subject:
Let’s stay with the topic and not go off at a tangent.
2 . technical a straight line that touches the outside of a curve but does not cut across it