zh.wikipedia.org/wiki/數值微分
数值微分是数值方法中的名词，是用函数的值及其他已知资讯来估计一函数导数的算法。
http://mathworld.wolfram.com/NumericalDifferentiation.html
Numerical differentiation is the process of finding the numerical value of a derivative of a given function at a given point. In general, numerical differentiation is more difficult than numerical integration. This is because while numerical integration requires only good continuity properties of the function being integrated, numerical differentiation requires more complicated properties such as Lipschitz classes. Numerical differentiation is implemented as ND[f, x, x0, Scale -> scale] in the Wolfram Language package NumericalCalculus` .