A **critical point** is a point on a graph at which the derivative is either equal to zero or does not exist.

If a critical point is equal to zero, it is called a stationary point (where the slope of the original graph is zero). If it does not exist, this can correspond to a discontinuity in the original graph or a vertical slope.

For functions of a single variable, critical points satisfy

For functions of multiple variables, critical points satisfy

## Properties

A critical point equal to zero may indicate the presence of an extreme value, if the second derivative of the function is non-zero. A positive second derivative indicates a local minima, and a negative second derivative indicates a local maxima. Note that some functions (e.g. or ) have critical points that aren't extreme values.