AP Calculus AB and BC glossary
Secant line
A secant line is a straight line through two points on a curve. Its slope equals the average rate of change between those points, and as the two points slide together the secant line approaches the tangent line.
The secant line is the concrete object behind the definition of the derivative. Watching its slope stabilize as the second point approaches the first is the clearest way to see why the derivative is defined with a limit.
You can move a point and watch this happen directly in the secant to tangent visualizer, where shrinking the gap turns the secant into the tangent in real time.
Appears in: Unit 2: Defining the Derivative