Any straight line which meets a particular geometrical figure at a point, just touching the figure without intersecting it, is called the tangent to the geometrical figure at that point.
For example, suppose we have a circle as shown in the image below. The line AB is a tangent because it touches but does not intersect the circle, while the line CD is not a tangent since it intersects the circle at point C.
|Tangent to a circle|
|Tangent to a parabola|