In the Wolfe inexact line search, why do we call the following condition as ``curvature condition" $$d_k^T \nabla f(x_k + \alpha_k d_k) \ge c_2 d_k^T \nabla f(x_k)?$$
I was trying to get an explanation through Hessian but failed. Do you have any answer why we refer the above condition as ``curvature condition"?