/'tændʒənt; `tændʒənt/
n
1 (geometry 几) straight line that touches the outside of a curve but does not cross it 切线. =>illus at circle 见circle插图.2 (abbr 缩写 tan) (mathematics 数) (in a right-angled triangle) ratio of the sides opposite and adjacent to a given angle 正切. Cf 参看 cosine, sine.
3 (idm 习语) go/fly off at a `tangent change suddenly from one line of thought, action, etc to another 突然改变思想、行动等.

n[C]
[Date: 1500-1600; Language: Latin;Origin: , present participle of tangere; TANGIBLE]
go off at a tangent
BrEgo off on a tangentAmE
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.
technical a straight line that touches the outside of a curve but does not cut across it