The answer depends on the context. In the context of a curve, a tangent is a straight line that touches the line without intersecting it. The antonym does not have a specific name because it could be a straight line that does not meet the curve at all, or it could be one that crosses the curve. Note that even a tangent can cross the curve at some other [distant] point(s).
a tangent
A common tangent is a line which is tangent to two (or more) curves.
31 degrees
45 degrees
tangent market
The tangent of 0.47 radians is about 0.508. The tangent of 0.47 degrees is about 0.00820.
Reciprocal of tangent is '1 /tangent' or ' Cosine / Sine '
196-164/2= 16
Tangent is 0.5317
Tangent 39 = 0.809784033195
tangent(450) = 0.935809013
236-124/2=56 degrees