Directional derivative
= Directional derivative
{title2=$D_vf$}
{wiki}
The directional derivative of $f$ at $x$ in the direction $v$ is
$$
D_vf(x)=\left.\frac d{dt}f(x+tv)\right|_{t=0}.
$$
For a differentiable scalar function on Euclidean space, it equals $v\mathbin\cdot\nabla f(x)$.