Tangent
A Tangent is a line that touches a curve at one point without cutting across it.

Tanned when she gently (tangent) touched the circle.
Example 1
The tangent touches the circle at one point – a tangent.

Example 2

Although the red line is curved in several ways, the blue line is still a tangent to the red line.
Example 3

The tangent meets the line on the graph at point a.