Directional Derivative
The directional derivative of a multivariable function is the rate of change of a function $f(x,y)$ at the direction of a unit vector $u$. In order to calculate its directional derivative, we also need its gradient. After knowing it, we can simply calculate the dot product of the two Vectors and that gives us that function’s directional derivative at $(x,y)$ with direction $u$:
$$D_u(x,y) = \triangledown f(x,y) \cdot u$$