For questions about secant lines, which are lines that pass through two points on some curve.
In geometry, a secant line of a curve is a line that (locally) intersects two points on the curve. A chord is an interval of a secant line, the portion of the line that lies within the curve. The word secant comes from the Latin word secare, meaning to cut.
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). 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.
