# Secant line

For information on the secant trigonometric function, see Trigonometric functions.
Common lines and line segments on a circle, including the secant line

In geometry, a secant line of a curve is a line that (locally) intersects two points on the curve.[1] A chord is an interval of a secant line, the portion of the line that lies within the curve.[2] The word secant comes from the Latin word secare, meaning to cut.[3]

Secants 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 that the first derivative of the curve is continuous at point P so that there is only one tangent).[1] As a consequence, one could say that the limit as Q approaches P 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.