There's three definitions.
noun
1.
a straight line or plane that touches a curve or curved surface at a point, but if extended does not cross it at that point.
2.
a completely different line of thought or action.
"he quickly went off on a tangent about wrestling"
adjective
(of a line or plane) touching, but not intersecting, a curve or curved surface.
"this curve is tangent to the average cost curve"
