directional derivative

A function which measures the rate of change of some other function at any point in a particular direction; formally, the directional derivative of a scalar function f( mathbf x)=f(x_1,x_2,…,x_n) along a vector mathbf v=(v_1,…,v_n) is the function ∇_(mathbf v)f defined by the limit ∇_(mathbf v)f( mathbf x)= lim _(h→0)(f( mathbf x+h mathbf v)-f( mathbf x))/h.