tan‧gent /ˈtændʒənt/ 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

[TahlilGaran] Dictionary of Contemporary English ▲