Tangent

In trigonometry, the tangent is defined as a relationship between the sine and cosine functions:

$\tan \theta ={\frac {\sin \theta }{\cos \theta }}=\cot \left({\frac {\pi }{2}}-\theta \right)={\frac {1}{\cot \theta }}\,$

Lines which intersect a circle or curve at only one, unique point are said to be tangent to that circle or curve.