tangent

Noun B2 – Upper-Intermediate

A straight line that touches a curve at exactly one point without crossing it, or, in trigonometry, the ratio of the opposite side to the adjacent side in a right triangle.

"The engineer drew a tangent to the curve to find the object's instantaneous direction of travel."

Informally, "go off on a tangent" means to suddenly start talking about something unrelated to the main topic.