A tangent to a circle is a straight line that touches the circle at exactly one point and is perpendicular to the radius drawn to that point.
In geometry, a tangent (or tangent line) to a curve is a straight line that touches but does not intersect the curve. It can be defined as a line through a pair of infinitely close points on a curve.
If you zoom in to the point of tangency on the curve, the curve starts to look more and more like a straight line, and the tangent line becomes indistinguishable from the curve itself.