Directional derivative

Instantaneous rate of change of the function

In multivariable calculus, the directional derivative measures the instantaneous rate at which a function changes along a specified vector through a given point. If the vector is multiplied by a scalar, the corresponding directional derivative is multiplied by the same scalar. Some elementary texts instead use the phrase "directional derivative in the direction of v" for the rate of change of a function per unit distance in that direction.

From Wikipedia, under CC BY-SA. More on occurri.