secant, in mathematics. 1 In geometry, a secant is a straight line cutting a curve or surface. If it intersects the curve in two different points, as in the secant of a circle, the segment of the secant between the points is called a chord. The limiting position of a secant, if such a limit exists, is called the tangent to the curve or surface at that point. 2 A trigonometric function. See trigonometry.