The secant line (or simply the secant) of a curve is a straight line intersecting the curve in at least two distinct points. [The name is initially a participial form of the Latin verb secare `to cut'.]

If one sets a secant e.g. to the ``cubic parabola'' through its points and , there is also a third common point .

Notice that a secant line can also be tangent to the curve at some point, given that tangency is only a local property. In the following picture, is a secant line for the curve (since it intersects the curve at points and ), yet it is also a tangent line at the point .