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A secant line of a curve is a line that (locally) intersects two points on the curve. The word secant comes from the Latin secare, for to cut.

It can be used to approximate the tangent to a curve, at some point P. If the secant to a curve is defined by two points, P and Q, with P fixed and Q variable, as Q approaches P along the curve, the direction of the secant approaches that of the tangent at P, assuming there is just one. As a consequence, one could say that the limit of the secant's slope, or direction, is that of the tangent. In calculus, this idea is the basis of the geometric definition of the derivative. A chord is the portion of a secant that lies within the curve.

The secant can be calculated as 1 / cos θ

Function	Abbreviation	Identities (using radians)
Sine	sin	${\displaystyle \sin \theta \equiv \cos \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\csc \theta }}\,}$
Cosine	cos	${\displaystyle \cos \theta \equiv \sin \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\sec \theta }}\,}$
Tangent	tan (or tg)	${\displaystyle \tan \theta \equiv {\frac {\sin \theta }{\cos \theta }}\equiv \cot \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\cot \theta }}\,}$
Cosecant	csc (or cosec)	${\displaystyle \csc \theta \equiv \sec \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\sin \theta }}\,}$
Secant	sec	${\displaystyle \sec \theta \equiv \csc \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\cos \theta }}\,}$
Cotangent	cot (or ctg or ctn)	${\displaystyle \cot \theta \equiv {\frac {\cos \theta }{\sin \theta }}\equiv \tan \left({\frac {\pi }{2}}-\theta \right)\equiv {\frac {1}{\tan \theta }}\,}$

• Curves
• Trigonometry