In geometry, a secant of a curve is a line that (locally) intersects two points on the curve. A chord is the interval of a secant that lies between the points at which it intersects the curve. The word secant comes from the Latin word secare, meaning to cut.
A secant may 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). 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.
- Elliptic curve, a curve for which every secant has a third point of intersection, from which a group law may be defined
- Quadrisecant, a line that intersects four points of a curve
- Secant plane, the three-dimensional equivalent of a secant line
- Secant variety, the union of secant lines and tangent lines to a given projective variety
- Protter, Murray H.; Protter, Philip E. (1988), Calculus with Analytic Geometry, Jones & Bartlett Learning, p. 62, ISBN 9780867200935.
- Gullberg, Jan (1997), Mathematics: From the Birth of Numbers, W. W. Norton & Company, p. 387, ISBN 9780393040029.
- Redgrove, Herbert Stanley (1913), Experimental Mensuration: An Elementary Test-book of Inductive Geometry, Van Nostrand, p. 167.