Descent direction ($\nabla f(x{)}^{T}s<0$)

A vector $s$ is a descent direction for a differentiable function $f$ at $x$ if its directional derivative $\nabla f(x{)}^{T}s$ is negative. The first-order expansion $f(x+\alpha s)=f(x)+\alpha \nabla f(x{)}^{T}s+o(\alpha )$ then gives $f(x+\alpha s)<f(x)$ for sufficiently small positive $\alpha$. The negative gradient is a descent direction whenever the gradient is nonzero.