tangent

Universal Words noun 1 (geometry) a straight line that touches the outside of a curve but does not cross it 2 (abbreviation tan) (mathematics) the RATIO of the length of the side opposite an angle in a RIGHT-ANGLED triangle to the length of the side next to it compareCOSINE, SINE IDIOMS fly / go off at a tangent (BrE) (AmE go off on a tangent) (informal) to suddenly start saying or doing sth that does not seem to be connected to what has gone before: He never sticks to the point but keeps going off at a tangent.